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PS#1 is the original change, the latest PS disables assembly
for MSAN and fixes up the .gitignore file.

-----------[Original Description Below]-----------
We're planning to experiment with av1 decoders, so this change
adds support for dav1d in Chromium.

This sets up the dav1d repo per modern 3rd party style: It's a
read-only clone of dav1d's GitHub mirror with the build config
and helper scripts living in the main Chromium src.git repo.
Due to this configuration, no local patches can be made.

Two helper scripts have been added "generate_configs.py" and
"generate_source.py". The former requires a few dependencies
spelled out in the README.chromium. The latter works simply
by globbing some source patterns into GN variables.

Thus far it seems all posix configurations (mac, linux, CrOS)
can share the same config, so only support linux and windows
in x86, x64 configurations has been added.

ARM support is not enabled yet since we haven't enabled libaom
for ARM and similarly dav1d is lacking much of the SIMD code.

To wire everything up this also:
- Adds a media::VideoDecoder based implementation and unit test.
- Adds a disabled-by-default Dav1dVideoDecoder base::Feature.

TBR=jam,liberato,mmoroz

Bug: 924370, 928357
Change-Id: I0e921599830d6e52019106e3ad42672873c94ece
Reviewed-on: https://chromium-review.googlesource.com/c/1452782Reviewed-by: Max Moroz <mmoroz@chromium.org>
Reviewed-by: Frank Liberato <liberato@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: John Abd-El-Malek <jam@chromium.org>
Auto-Submit: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628921}