Make webkitCancelFullScreen() an alias of webkitExitFullscreen()

It appears to be an accident of history that webkitCancelFullScreen()
behaves differently from webkitExitFullscreen(), as it was implemented
before the concept of a fullscreen element stack.

In Gecko, mozCancelFullScreen() pops only one element from the stack.

In the spec, "fully exit fullscreen" is now reserved for user escape
and page navigation, so any other instances should be changed where
possible.

There was no test coverage for the distinction, and it's very unlikely
that Web content could meaningfully depend on it.

BUG=399556

Review URL: https://codereview.chromium.org/434903002

git-svn-id: svn://svn.chromium.org/blink/trunk@179656 bbb929c8-8fbe-4397-9dbb-9b2b20218538