The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3d3dfcd34e38fcecaa1ff14bcd12cbc98259f36c

commit 3d3dfcd34e38fcecaa1ff14bcd12cbc98259f36c
Author: zqzhang <zqzhang@chromium.org>
Date: Thu Mar 23 21:07:02 2017

Fix AutoplayUmaHelper when autoplay is initiated from multiple sources

This CL makes AutoplayUmaHelper to handle the case where the page
autoplays by both attribute and play() method. Therefore we can avoid
counting the metrics into the wrong bucket.

BUG= 699176 

Review-Url: https://codereview.chromium.org/2767093002
Cr-Commit-Position: refs/heads/master@{#459215}

[modify] https://crrev.com/3d3dfcd34e38fcecaa1ff14bcd12cbc98259f36c/third_party/WebKit/Source/core/html/AutoplayUmaHelper.cpp
[modify] https://crrev.com/3d3dfcd34e38fcecaa1ff14bcd12cbc98259f36c/third_party/WebKit/Source/core/html/AutoplayUmaHelper.h
[modify] https://crrev.com/3d3dfcd34e38fcecaa1ff14bcd12cbc98259f36c/third_party/WebKit/Source/core/html/AutoplayUmaHelperTest.cpp
[modify] https://crrev.com/3d3dfcd34e38fcecaa1ff14bcd12cbc98259f36c/tools/metrics/histograms/histograms.xml
[modify] https://crrev.com/3d3dfcd34e38fcecaa1ff14bcd12cbc98259f36c/tools/metrics/rappor/rappor.xml