Introduce FullscreenElementStack::RequestType

This replaces flags + FullScreenCheckType, for a few reasons:

 1. An UnprefixedRequest will be added, for which neither
    ALLOW_KEYBOARD_INPUT nor FullScreenCheckType is possible. Not all
    previously possible combinations are possible, either.

 2. An enum is clearer than flags at the call site. In particular,
    0 was passed to HTMLMediaElement::enterFullscreen() which would
    disallow keyboard input, which was not obivous and possibly not
    even the original intention. The behavior was preserved but should
    probably be changed to allow keyboard input in a future patch.

 3. LEGACY_MOZILLA_REQUEST as a public part of Element.h is weird.

BUG=383813

Review URL: https://codereview.chromium.org/356933002

git-svn-id: svn://svn.chromium.org/blink/trunk@177061 bbb929c8-8fbe-4397-9dbb-9b2b20218538