Fix testVideoMetrics metric change after new ffmpeg release

Bug: 807049
Change-Id: Ia223724629db4f04a676a023a4dbda365a0dbba3
Reviewed-on: https://chromium-review.googlesource.com/893719Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Robert Ogden <robertogden@google.com>
Cr-Commit-Position: refs/heads/master@{#532962}

tools/chrome_proxy/webdriver/video.py

@@ -123,7 +123,7 @@ class Video(IntegrationTest):
   @Slow
   def testVideoMetrics(self):
     expected = {
-      'duration': 3.128,
+      'duration': 3.068,
       'webkitDecodedFrameCount': 54.0,
       'videoWidth': 1280.0,
       'videoHeight': 720.0