Replace drainUntilIdle with a dispatch_barrier_sync for Breakpad tests

ios/chrome/test/app/chrome_test_util.mm

Index: ios/chrome/test/app/chrome_test_util.mm
diff --git a/ios/chrome/test/app/chrome_test_util.mm b/ios/chrome/test/app/chrome_test_util.mm
index 9f43f5a729f2b27a9ccd6e1ef9523680d20880f8..0b8f1a327f8301f58dee92da00d4f2f9245dfd50 100644
--- a/ios/chrome/test/app/chrome_test_util.mm
+++ b/ios/chrome/test/app/chrome_test_util.mm
@@ -31,6 +31,7 @@
 @interface BreakpadController (Testing)
 - (BOOL)isEnabled;
 - (BOOL)isUploadingEnabled;
+- (dispatch_queue_t)queue;

[baxley, 2017/04/20 16:37:42]

Probably outside the scope of this CL... Do you have any thoughts on public APIs
we could use to achieve something similar? Or perhaps things we could make
public APIs?

 @end
 @implementation BreakpadController (Testing)
 - (BOOL)isEnabled {
@@ -39,6 +40,9 @@
 - (BOOL)isUploadingEnabled {
   return enableUploads_;
 }
+- (dispatch_queue_t)queue {
+  return queue_;
+}
 @end
 
 namespace {
@@ -201,6 +205,12 @@ bool IsFirstLaunchAfterUpgrade() {
   return [chrome_test_util::GetMainController() isFirstLaunchAfterUpgrade];
 }
 
+void WaitForBreakpadQueue() {
+  dispatch_queue_t queue = [[BreakpadController sharedInstance] queue];
+  dispatch_barrier_sync(queue, ^{
+                        });
+}
+
 void OpenChromeFromExternalApp(const GURL& url) {
   [[[UIApplication sharedApplication] delegate]
       applicationWillResignActive:[UIApplication sharedApplication]];