Enable PreloadDefaultIsMetadata by default.

Starting M64, set <video> and <audio> preload= default to metadata.

Intent to Implement and Ship:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/5CDvJkdxyQ8

Bug: 310450
Change-Id: I681f532a688646523028f8cc8578d8da29ea1881
Reviewed-on: https://chromium-review.googlesource.com/731675
Commit-Queue: apacible <apacible@chromium.org>
Reviewed-by: Dimitri Glazkov <dglazkov@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#511696}
18 files changed