Reconcile -[BreakpadController withBreakpadRef:] with its documentation.

The header states that if the controller is not -start:'ed that it will call
the block with a NULL BreakpadRef. As previously implemented, it asserted if
it was not started.

Change-Id: I3a329a773c0484dc1b74013717b68426758ea2cd
Reviewed-on: https://chromium-review.googlesource.com/829834
Reviewed-by: Mark Mentovai <mark@chromium.org>

src/client/ios/BreakpadController.mm

@@ -249,10 +249,8 @@ NSString* GetPlatform() {
 }
 
 - (void)withBreakpadRef:(void(^)(BreakpadRef))callback {
-  NSAssert(started_,
-      @"The controller must be started before withBreakpadRef is called");
   dispatch_async(queue_, ^{
-      callback(breakpadRef_);
+      callback(started_ ? breakpadRef_ : NULL);
   });
 }