Use dav1d by default

This CL turns on dav1d by default. It also disables libaom to save on
binary size.

Bug: 924370
Change-Id: I4d331a3b66fe6031930f5f56880866b9a3c92df8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1539901
Commit-Queue: Chrome Cunningham <chcunningham@chromium.org>
Auto-Submit: Thomas Guilbert <tguilbert@chromium.org>
Reviewed-by: Tom Finegan <tomfinegan@chromium.org>
Reviewed-by: Chrome Cunningham <chcunningham@chromium.org>
Cr-Commit-Position: refs/heads/master@{#645600}