Allow HTMLMediaElement.currentTime to be set before the transition to HAVE_METADATA

Spec now allows currentTime to be set when the readyState is still HAVE_NOTHING.
This patch modifies the behaviour in accordance to the spec. The default playback
start position is set to the new currentTime and the resource plays from that point
once it is loaded.
http://www.whatwg.org/specs/web-apps/current-work/#dom-media-currenttime

BUG=399314

Review URL: https://codereview.chromium.org/517593003

git-svn-id: svn://svn.chromium.org/blink/trunk@181736 bbb929c8-8fbe-4397-9dbb-9b2b20218538