Fix race conditions in window snapshot code.

tools/telemetry/telemetry/core/tab_unittest.py

 # Copyright (c) 2012 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.
 
 import logging
-import time
 
 from telemetry.core import util
 from telemetry.core import exceptions
 from telemetry.unittest import tab_test_case
-from telemetry.unittest import DisabledTest
 
 
 def _IsDocumentVisible(tab):
   state = tab.EvaluateJavaScript('document.webkitVisibilityState')
   # TODO(dtu): Remove when crbug.com/166243 is fixed.
   tab.Disconnect()
   return state == 'visible'
 
 
 class TabTest(tab_test_case.TabTestCase):
@@ skipping 28 matching lines @@
     self._tab.Activate()
     util.WaitFor(lambda: _IsDocumentVisible(self._tab), timeout=5)
     self.assertFalse(_IsDocumentVisible(new_tab))
 
 
 class GpuTabTest(tab_test_case.TabTestCase):
   def setUp(self):
     self._extra_browser_args = ['--enable-gpu-benchmarking']
     super(GpuTabTest, self).setUp()
 
-  @DisabledTest
   def testScreenshot(self):
     if not self._tab.screenshot_supported:
       logging.warning('Browser does not support screenshots, skipping test.')
       return
 
     self._browser.SetHTTPServerDirectories(util.GetUnittestDataDir())
     self._tab.Navigate(
       self._browser.http_server.UrlOf('green_rect.html'))
     self._tab.WaitForDocumentReadyStateToBeComplete()
     pixel_ratio = self._tab.EvaluateJavaScript('window.devicePixelRatio || 1')
 
-    # TODO(bajones): Sleep for a bit to counter BUG 260878.
-    time.sleep(0.5)
     screenshot = self._tab.Screenshot(5)
     assert screenshot
     screenshot.GetPixelColor(0 * pixel_ratio, 0 * pixel_ratio).AssertIsRGB(
         0, 255, 0, tolerance=2)
     screenshot.GetPixelColor(31 * pixel_ratio, 31 * pixel_ratio).AssertIsRGB(
         0, 255, 0, tolerance=2)
     screenshot.GetPixelColor(32 * pixel_ratio, 32 * pixel_ratio).AssertIsRGB(
         255, 255, 255, tolerance=2)
+
+  def testScreenshotSync(self):
+    if not self._tab.screenshot_supported:
+      logging.warning('Browser does not support screenshots, skipping test.')
+      return
+
+    self._browser.SetHTTPServerDirectories(util.GetUnittestDataDir())
+    self._tab.Navigate(
+      self._browser.http_server.UrlOf('screenshot_sync.html'))
+    self._tab.WaitForDocumentReadyStateToBeComplete()
+
+    def IsTestComplete():
+      return self._tab.EvaluateJavaScript('window.__testComplete')
+    util.WaitFor(IsTestComplete, 120)
+
+    message = self._tab.EvaluateJavaScript('window.__testMessage')
+    if message:
+      logging.error(message)
+    assert self._tab.EvaluateJavaScript('window.__testSuccess')