Add a custom error message in EG WaitUtil

ios/testing/earl_grey/wait_util.mm

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ios/testing/earl_grey/wait_util.h"
 
 #import <EarlGrey/EarlGrey.h>
 
 #include "base/test/ios/wait_util.h"
 
 namespace testing {
 
 const NSTimeInterval kSpinDelaySeconds = 0.01;
 const NSTimeInterval kWaitForJSCompletionTimeout = 2.0;
 const NSTimeInterval kWaitForUIElementTimeout = 4.0;
 
-void WaitUntilCondition(NSTimeInterval timeout, bool (^condition)(void)) {
+void WaitUntilCondition(NSTimeInterval timeout,
+                        NSString* timeoutDescription,
+                        bool (^condition)(void)) {
   NSDate* deadline = [NSDate dateWithTimeIntervalSinceNow:timeout];
   while (!condition() &&
          [[NSDate date] compare:deadline] != NSOrderedDescending) {
     base::test::ios::SpinRunLoopWithMaxDelay(
         base::TimeDelta::FromSecondsD(testing::kSpinDelaySeconds));
   }
-  GREYAssert(condition(), @"Timeout waiting for condition.");
+  GREYAssert(condition(), timeoutDescription);
+}
+
+void WaitUntilCondition(NSTimeInterval timeout, bool (^condition)(void)) {
+  WaitUntilCondition(timeout, @"Timeout waiting for condition.", condition);
 }
 
 }  // namespace testing