Remove the mediaEnabled setting

This setting only prevented HTMLMediaElement::load() from running, which
very rarely makes any difference, since scripts modifying the DOM will
almost always implicitly invoke resource selection. The exceptions are
adding source elements when networkState is not NETWORK_EMPTY, removing
the src attribute, and removing source elements.

Since the callers to setMediaEnabled(false) seems to have survived
without actually disabling media, just remove the setting for now.

BUG=none

Review URL: https://codereview.chromium.org/169363004

git-svn-id: svn://svn.chromium.org/blink/trunk@167575 bbb929c8-8fbe-4397-9dbb-9b2b20218538