Merge willEnter/willExitFullScreen into didEnter/didExitFullScreen

The will/did callback pairs came into being in 2011:
https://bugs.webkit.org/show_bug.cgi?id=70477
https://code.google.com/p/chromium/issues/detail?id=100264

The original problem was that the webkitfullscreenchange was
dispatched too early, but this is not affected since the did*
callbacks are the ones left in use.

The risk is that something done in the will* callbacks is too late if
done moved to the did* callbacks. That amounts to:

 - Saving placeholder style and unwrapping/wrapping the renderer in
   Fullscreen::willEnterFullScreenForElement().

 - The overlayFullscreenVideoEnabled() bits in
   HTMLMediaElement::willStopBeingFullscreenElement() and
   FullscreenController::willExitFullScreen().

Since the only difference between will* and did* is that the new size
is known and events aren't fired synchronously, this seems likely to
be safe.

BUG=407002

Review URL: https://codereview.chromium.org/497723002

git-svn-id: svn://svn.chromium.org/blink/trunk@181218 bbb929c8-8fbe-4397-9dbb-9b2b20218538
http/tests/serviceworker/request.html is flaky

BUG=409755
TBR=hclam@chromium.org

Review URL: https://codereview.chromium.org/522483004

git-svn-id: svn://svn.chromium.org/blink/trunk@181219 bbb929c8-8fbe-4397-9dbb-9b2b20218538