Make sure to fallback to VDA1 if mojo video decoder is disabled at buildtime

Also avoids a few ipc round trips when mojo video decoder is enabled
at buildtime but disabled at runtime.
Previously external_decoder_factory_->CreateVideoDecoders was still
called in that case.

Tested on Linux desktop with gn args:
  - use_vaapi = true (default is false)
  - enable_mojo_media = false (default is also false)
  ./out/release/chrome --ignore-gpu-blacklist --use-gl=desktop url_to_vp9_video
  ./out/release/chrome --ignore-gpu-blacklist --use-gl=egl url_to_vp9_video
  -> GpuVideoDecoder (VDA1) was in use

Bug: 522298
Change-Id: Ia7566877f76da8d108d8993c57c6d29d745cfa8a
Reviewed-on: https://chromium-review.googlesource.com/c/1374741
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Julien Isorce <julien.isorce@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616855}
2 files changed