FWIW, the OverlayPlayButton is rarely displayed according to our metrics. I think this is a side effect of us recording the metrics only when the metadata are loaded which means that the button may be visible more often and the click ratio will be exaggerated.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/6fdc60f92edd50012f489ba52f18bcdad26b3794

commit 6fdc60f92edd50012f489ba52f18bcdad26b3794
Author: Mounir Lamouri <mlamouri@chromium.org>
Date: Wed Nov 15 19:26:31 2017

Media Controls: fix issues with CTR metrics calculation.

There are 3 fixes:
 - Record visibility of CastOverlayButton;
 - Fix type in ClosedCaptionButton string;
 - Record clicks for PlayOverlayButton.

Bug:  785295 
Change-Id: I2b57bbef248666aaa3683cd14907043ecf31aea3
Reviewed-on: https://chromium-review.googlesource.com/771795
Reviewed-by: Becca Hughes <beccahughes@chromium.org>
Commit-Queue: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516781}
[modify] https://crrev.com/6fdc60f92edd50012f489ba52f18bcdad26b3794/third_party/WebKit/Source/modules/media_controls/MediaControlsImpl.cpp
[modify] https://crrev.com/6fdc60f92edd50012f489ba52f18bcdad26b3794/third_party/WebKit/Source/modules/media_controls/elements/MediaControlOverlayPlayButtonElement.cpp
[modify] https://crrev.com/6fdc60f92edd50012f489ba52f18bcdad26b3794/third_party/WebKit/Source/modules/media_controls/elements/MediaControlToggleClosedCaptionsButtonElement.cpp