Create EarlGrey wait utility.

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;
-const NSTimeInterval kWaitForJSCompletionTimeout = 2.0;
+
+void WaitUntilCondition(NSTimeInterval timeout, 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.");
 }
+
+}  // namespace testing