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: I67a1c6828e94a6f82688755747f4122ac3f0bf47
Reviewed-on: https://chromium-review.googlesource.com/596623
Commit-Queue: apacible <apacible@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Reviewed-by: Dimitri Glazkov <dglazkov@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#497864}
10 files changed