Cleanup and rename DocumentEventQueue::enqueueOrDispatchScrollEvent to enqueueScrollEvent

This method never actually fires an event, it always enqueues one so we can rename it.
We can also detect if the target is a Document by using isDocument instead of having
the extra enum argument. I also changed it so that we don't create a new Event only
to discard it when one is already scheduled.

BUG=299317

Review URL: https://chromiumcodereview.appspot.com/24928002

git-svn-id: svn://svn.chromium.org/blink/trunk@158481 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove custom getter for Clipboard::types().

Also fixes an old bug where we would return null instead of an empty
list and another bug that prevents RVO from being applied.

BUG=226785,226903

Review URL: https://chromiumcodereview.appspot.com/24936002

git-svn-id: svn://svn.chromium.org/blink/trunk@158482 bbb929c8-8fbe-4397-9dbb-9b2b20218538