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
Committed:
https://crrev.com/8acbe326325ecfef82cd5d98b9ffd5f28e17ff76
Cr-Commit-Position: refs/heads/master@{#427658}