Added a new runtime setting for RequireUserGestureForFullscreenRestriction.

Control the restriction requiring user gesture for the media element to enter
fullscreen with the new mediaFullscreenRequiresUserGesture setting.

BUG=288678

Review URL: https://chromiumcodereview.appspot.com/23440027

git-svn-id: svn://svn.chromium.org/blink/trunk@157643 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Ignore should-scroll-on-main-thread if main frame is not scrollable

This patch was originally reverted because the call to FrameView::isScrollable
that was added could crash if the body had a NULL style. This patch adds a
fix handle that scenario, and a test that reproduces it.

This reverts commit 0e53aff60891596f88df001ef3d91422d83cc560.
(Review URL: https://chromiumcodereview.appspot.com/23477042)

And relands commit 1a3edc542e31626a9b46ae2b18eb227c7031f3d6
(Review URL: https://chromiumcodereview.appspot.com/22694003)

BUG=280679

Review URL: https://chromiumcodereview.appspot.com/23641004

git-svn-id: svn://svn.chromium.org/blink/trunk@157644 bbb929c8-8fbe-4397-9dbb-9b2b20218538