Check for valid webMediaPlayer() before using it.

Check for valid webMediaPlayer() before using it.

This fixes the underlying issue in bug 456312 and reverts the revert in
https://codereview.chromium.org/905023002, adding the fix and the layout tests back.

Manually tested by visiting youtube.com and clicking on videos, before the current video is finished. This requires the Audio EQ (HTML5 Audio Equalizer for Chrome) extension to be added and enabled.

BUG=456312, 313939
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=190367

[Raymond Toy, 2015-02-09 18:02:56]

rtoy@chromium.org changed reviewers:
+ hongchan@chromium.org, kbr@chromium.org

[Raymond Toy, 2015-02-09 18:02:56]

PTAL.

[Ken Russell (switch to Gerrit), 2015-02-10 16:12:56]

LGTM overall, but could you please try to add tests that catch the previous
crash? Can you provoke it by just not playing the media element?

[Raymond Toy, 2015-02-17 22:15:30]

On 2015/02/10 16:12:56, Ken Russell wrote:
> LGTM overall, but could you please try to add tests that catch the previous
> crash? Can you provoke it by just not playing the media element?

Not playing the element doesn't invoke the issue. I tried to duplicate some of
what youtube and AudioEQ do, but have not figured out exactly what causes the
webMediaPlayer to be 0 for some period of time.

Issue filed to create a test for this:
https://code.google.com/p/chromium/issues/detail?id=459362

[Raymond Toy, 2015-02-17 22:16:17]

The CQ bit was checked by rtoy@chromium.org

[commit-bot: I haz the power, 2015-02-17 22:16:50]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/905393002/1

[commit-bot: I haz the power, 2015-02-17 23:49:56]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=190367