Remove the user gesture restriction for HTMLMediaElement::pause()

This is very nearly dead code, because when the setting is enabled, the
only way to play a video is using play(), which removes all restrictions
if it succeeds. Therefore, the only observable behavior from the pause
restriction is that if pause() is called from an input event handler
when networkState is NETWORK_EMPTY, LoadMediaResource is scheduled.

Further, it hardly seems like a useful restriction even if it worked.
Web sites that pause a video against the will of the user seem rather
improbable. Possibly the restriction was originally added to guard the
scheduling of LoadMediaResource.

Preventing LoadMediaResource is not useful, since there are several
other ways to trigger a load, e.g. just <video src="foo"> in markup.

Note that there was no test for this restriction, but there are tests
for load(), play() and autoplay.

BUG=342339

Review URL: https://codereview.chromium.org/130813006

git-svn-id: svn://svn.chromium.org/blink/trunk@166888 bbb929c8-8fbe-4397-9dbb-9b2b20218538