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This CL adds SpatialLayer to VideoEncodeAccelerator::Config so
that a VEA client is able to configure a spatial layer encoding.
The configuration is useful to configure temporal layer encoding
too. Supporting temporal layer with vp9 hw encoder is a short
term goal today.

Design doc: https://docs.google.com/document/d/1yeCV36yhk9zna4qwngPiJGhSUkDyJHRlfEpziFgJfHo#heading=h.aeh6kjktcuzy

Bug: 1030199
Test: media_unittests
Change-Id: I1e0524a71b043ccfa0e41d08e8de4b86e9c17c9e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2131444
Commit-Queue: Hirokazu Honda <hiroh@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756300}