Remove newMediaPlaybackUi flag from content/ and Blink.

The flag has been enabled by default for a while and makes the code more
complex. We no longer test the old code path which is likely broken at
this point.

A follow-up CL will remove the files that are now obsolete.

BUG=658520
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2447503002
Cr-Commit-Position: refs/heads/master@{#427658}
19 files changed