Added WARN_UNUSED_RESULT annotation to WaitUntilConditionOrTimeout.

Ignoring the result of WaitUntilConditionOrTimeout call is incorrect in
many cases. CL adds WARN_UNUSED_RESULT annotation to
WaitUntilConditionOrTimeout to force callers to do the checks.

Bug: 780062
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I96daa8dfc6ec280d7f622a91f474714fe48fe3bf
Reviewed-on: https://chromium-review.googlesource.com/748108Reviewed-by: Mike Baxley <baxley@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513277}

ios/testing/wait_util.h

@@ -7,6 +7,7 @@
 
 #import <Foundation/Foundation.h>
 
+#include "base/compiler_specific.h"
 #import "base/ios/block_types.h"
 
 namespace testing {
@@ -29,7 +30,7 @@ extern const NSTimeInterval kWaitForPageLoadTimeout;
 // Returns true when condition() becomes true, otherwise returns false after
 // |timeout|.
 bool WaitUntilConditionOrTimeout(NSTimeInterval timeout,
-                                 ConditionBlock condition);
+                                 ConditionBlock condition) WARN_UNUSED_RESULT;
 
 }  // namespace testing