Don't delay loading if autoplay is set and preload equals 'none'.

This fixes a regression that was accidentally introduced in r169669.
If autoplay is set, then loading should not be delayed even if preload
was set to "none".

BUG=372770, 378682
TEST=LayoutTests/media/autoplay-with-preload-none.html

Review URL: https://codereview.chromium.org/313963004

git-svn-id: svn://svn.chromium.org/blink/trunk@175517 bbb929c8-8fbe-4397-9dbb-9b2b20218538