Create EarlGrey wait utility.

ios/testing/earl_grey/wait_util.h

 // 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.
 
 #ifndef IOS_TESTING_EARL_GREY_WAIT_UTIL_H_
 #define IOS_TESTING_EARL_GREY_WAIT_UTIL_H_
 
 #import <Foundation/Foundation.h>
 
 namespace testing {
 
 // Constant for UI wait loop in seconds.
 extern const NSTimeInterval kSpinDelaySeconds;
 
 // Constant for timeout in seconds while waiting for UI element.
 extern const NSTimeInterval kWaitForUIElementTimeout;
 
 // Constant for timeout in seconds while waiting for JavaScript completion.
 extern const NSTimeInterval kWaitForJSCompletionTimeout;
-}
+
+// Waits until |condition| is true, or induces GREYAssert after |timeout|.
+void WaitUntilCondition(NSTimeInterval timeout, bool (^condition)(void));
+
+}  // namespace testing
 
 #endif  // IOS_TESTING_EARL_GREY_WAIT_UTIL_H_