Remove traces of beforeload in HTMLMediaElement

There now remain no places in HTMLMediaElement where events are
dispatched synchronously, so there's no need to protect(this). For the
same reason, the ASSERT is irrelevant, even if it holds true.

BUG=none

Review URL: https://codereview.chromium.org/213343002

git-svn-id: svn://svn.chromium.org/blink/trunk@170123 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Add a UseCounter for the webkitallowfullscreen attribute

We support the unprefixed allowfullscreen attribute, so it would be nice
to also reflect it in HTMLIFrameElement.allowFullscreen. However, that's
tricky while there are two backing attributes, so first measure if the
prefixed attribute can be removed yet.

BUG=none

Review URL: https://codereview.chromium.org/212433004

git-svn-id: svn://svn.chromium.org/blink/trunk@170124 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove unneeded mutable keyword in some member variables

Some member variables have mutable keyword though they don't used by const function.
This cl removes them.

BUG=N/A

Review URL: https://codereview.chromium.org/212183004

git-svn-id: svn://svn.chromium.org/blink/trunk@170126 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Mark some tests flaky

BUG=356991, 356992, 356994, 356995, 356996, 356997
TBR=enne@chromium.org

Review URL: https://codereview.chromium.org/214213002

git-svn-id: svn://svn.chromium.org/blink/trunk@170127 bbb929c8-8fbe-4397-9dbb-9b2b20218538