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

load() resets some state and invokes resource selection. Preventing
resource selection is not useful, since there are several other
unguarded ways to trigger it using plain markup or scripts.

m_loadInitiatedByUserGesture has been unused outside of load() since the
npr.org quirk was removed in r158703.

If a similar feature is needed in the future, forcing preload="none"
internally would be an alternative.

BUG=342339

Review URL: https://codereview.chromium.org/144663006

git-svn-id: svn://svn.chromium.org/blink/trunk@166895 bbb929c8-8fbe-4397-9dbb-9b2b20218538