Allow load if play() is immediately called when preload == 'none'

This fixes a bug where the load gets blocked by preload == 'none'
eventhough JavaScript has called play() after setting the src
attribute. This particular bug only appears if play() is called before
the JavaScript code relinquishes control back to the UA.

Tests cases were added to make sure that immediate load() and play()
calls both trigger loading. Before this change, only immediate load()
calls ignored the preload == "none" state.

BUG=177870
TEST=LayoutTest/media/video-preload.html

Review URL: https://codereview.chromium.org/313173008

git-svn-id: svn://svn.chromium.org/blink/trunk@175590 bbb929c8-8fbe-4397-9dbb-9b2b20218538