Fix race conditions in window snapshot code.

tools/telemetry/telemetry/core/tab_unittest.py

Index: tools/telemetry/telemetry/core/tab_unittest.py
diff --git a/tools/telemetry/telemetry/core/tab_unittest.py b/tools/telemetry/telemetry/core/tab_unittest.py
index f555dc2812745a06c927cc1517a102b31bfcc07b..2258e76960a288b66b2ca5eb2ee2462a82b90156 100644
--- a/tools/telemetry/telemetry/core/tab_unittest.py
+++ b/tools/telemetry/telemetry/core/tab_unittest.py
@@ -3,12 +3,10 @@
 # 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):
@@ -56,7 +54,6 @@ class GpuTabTest(tab_test_case.TabTestCase):
     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.')
@@ -68,8 +65,6 @@ class GpuTabTest(tab_test_case.TabTestCase):
     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(