There is no need to use the main queue just for perform selector.

We were using the main queue to queue up a perform selector and then the code
[self sendStoredCrashReports] was immediately doing a dispatch_async.
This unnecessary thread switching is not needed.

We simplify the above logic and use dispatch_after to queue the block on
the
internal queue after a delay

Note that main queue is typically more loaded and it is better for
non-UI code
to not use the main queue. This may also help improve crash log upload.

This change also switches from @synchronized to dispatch_once as that is
faster
Reference:
http://googlemac.blogspot.com/2006/10/synchronized-swimming.html

BUG=

Change-Id: I81035149cbbf13a3058ca3a11e6efd23980f19ad
Reviewed-on: https://chromium-review.googlesource.com/441364
Reviewed-by: Joshua Peraza <jperaza@chromium.org>

src/client/ios/BreakpadController.mm

@@ -93,11 +93,12 @@ NSString* GetPlatform() {
 @implementation BreakpadController
 
 + (BreakpadController*)sharedInstance {
-  @synchronized(self) {
-    static BreakpadController* sharedInstance_ =
-        [[BreakpadController alloc] initSingleton];
-    return sharedInstance_;
-  }
+  static dispatch_once_t onceToken;
+  static BreakpadController* sharedInstance ;
+  dispatch_once(&onceToken, ^{
+      sharedInstance = [[BreakpadController alloc] initSingleton];
+  });
+  return sharedInstance;
 }
 
 - (id)init {
@@ -179,10 +180,9 @@ NSString* GetPlatform() {
         enableUploads_ = YES;
         [self sendStoredCrashReports];
       } else {
+        // disable the enableUpload_ flag.
+        // sendDelay checks this flag and disables the upload of logs by sendStoredCrashReports
         enableUploads_ = NO;
-        [NSObject cancelPreviousPerformRequestsWithTarget:self
-            selector:@selector(sendStoredCrashReports)
-            object:nil];
       }
   });
 }
@@ -317,38 +317,35 @@ NSString* GetPlatform() {
   [userDefaults synchronize];
 }
 
+// This method must be called from the breakpad queue.
 - (void)sendStoredCrashReports {
-  dispatch_async(queue_, ^{
-      if (BreakpadGetCrashReportCount(breakpadRef_) == 0)
-        return;
+  if (BreakpadGetCrashReportCount(breakpadRef_) == 0)
+    return;
 
-      int timeToWait = [self sendDelay];
+  int timeToWait = [self sendDelay];
 
-      // Unable to ever send report.
-      if (timeToWait == -1)
-        return;
+  // Unable to ever send report.
+  if (timeToWait == -1)
+    return;
 
-      // A report can be sent now.
-      if (timeToWait == 0) {
-        [self reportWillBeSent];
-        BreakpadUploadNextReportWithParameters(breakpadRef_,
-                                               uploadTimeParameters_);
+  // A report can be sent now.
+  if (timeToWait == 0) {
+    [self reportWillBeSent];
+    BreakpadUploadNextReportWithParameters(breakpadRef_,
+                                           uploadTimeParameters_);
 
-        // If more reports must be sent, make sure this method is called again.
-        if (BreakpadGetCrashReportCount(breakpadRef_) > 0)
-          timeToWait = uploadIntervalInSeconds_;
-      }
+    // If more reports must be sent, make sure this method is called again.
+    if (BreakpadGetCrashReportCount(breakpadRef_) > 0)
+      timeToWait = uploadIntervalInSeconds_;
+  }
 
-      // A report must be sent later.
-      if (timeToWait > 0) {
-        // performSelector: doesn't work on queue_
-        dispatch_async(dispatch_get_main_queue(), ^{
-            [self performSelector:@selector(sendStoredCrashReports)
-                       withObject:nil
-                       afterDelay:timeToWait];
-        });
-     }
-  });
+  // A report must be sent later.
+  if (timeToWait > 0) {
+    dispatch_time_t delay = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(timeToWait * NSEC_PER_SEC));
+    dispatch_after(delay, queue_, ^{
+        [self sendStoredCrashReports];
+    });
+  }
 }
 
 @end