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 3bc3089104cdbb171d09cea3fb34bfa45b53e596..b47cd44433bcd672371b4d860a152f16b5d2568d 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):
@@ -57,7 +55,6 @@ 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.')
@@ -69,8 +66,6 @@ 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(
@@ -79,3 +74,22 @@ class GpuTabTest(tab_test_case.TabTestCase):
         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')