Fix HTMLVideoElement aspect ratio on a failed poster load.

This fixes an aspect ratio change triggered by a failed poster URL
load. A failed load was causing the element aspect ratio to be
changed to 1x1 instead of keeping the default aspect ratio that would
be used if a poster URL load was never even attempted.

Added tests to verify aspect ratio behaviour for successful & failed
poster loads.

BUG=328865
TESTS=LayoutTests/media/video-poster.html

Review URL: https://codereview.chromium.org/106883009

git-svn-id: svn://svn.chromium.org/blink/trunk@164643 bbb929c8-8fbe-4397-9dbb-9b2b20218538