Start timeline recording using TimelinePanel methods instead of agent methods

We are going to switch to tracing backend for the timeline and there won't be TimelineAgent so the tests are changed to use higher level abstractions.

BUG=398785

Review URL: https://codereview.chromium.org/480003002

git-svn-id: svn://svn.chromium.org/blink/trunk@180438 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove webkitIsFullScreen everywhere except in bindings

It's equivalent to webkitCurrentFullScreenElement != null.

BUG=383813

Review URL: https://codereview.chromium.org/476353002

git-svn-id: svn://svn.chromium.org/blink/trunk@180439 bbb929c8-8fbe-4397-9dbb-9b2b20218538