Merge the fullscreen event queues

The two queues are both processed by a single timer, so they might just
as well be a single queue, which also avoids looping over each queue
separately.

Instead of queuing Nodes, Events are created and queued. This means that
the event type is made explicit, which will make it simpler to support
both prefixed and unprefixed events using the same code.

BUG=383813

Review URL: https://codereview.chromium.org/365903006

git-svn-id: svn://svn.chromium.org/blink/trunk@177509 bbb929c8-8fbe-4397-9dbb-9b2b20218538