Drop event should generate user gesture.

Generate a user gesture when drop event is fired. This is useful for extensions
to call some user gesture based APIs during a drop event. Please see the bug
for a sample extension.

R=adamk@chromium.org,abarth@chromium.org
BUG=355269

Review URL: https://codereview.chromium.org/208853006

git-svn-id: svn://svn.chromium.org/blink/trunk@170079 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Make mediaPlayerRequestFullscreen() user gesture bypass explicit

mediaPlayerRequestFullscreen() is only ever called from Chromium's
BrowserMediaPlayerManager::OnProtectedSurfaceRequested() when the
--disable-gesture-requirement-for-media-fullscreen flag is used.

Make this behavior explicit, limiting it to just this case instead of
disabling it even for script-initiated fullscreen requests.

This will allow removing the mediaFullscreenRequiresUserGesture setting
once Chromium has been updated.

BUG=355439
TEST=LayoutTests/media/media-player-request-fullscreen.html

Review URL: https://codereview.chromium.org/209693003

git-svn-id: svn://svn.chromium.org/blink/trunk@170080 bbb929c8-8fbe-4397-9dbb-9b2b20218538