Fall back to sw video decoding for sw compositing.

VideoResourceUpdater can't currently convert hw VideoFrame resources
into sw planes in sw comspositing mode, since it isn't given a
context provider to use for the conversion.  This CL causes it to
fall back to software video decoding in that case.

Note that adding support to VideoResourceUpdater isn't terribly
difficult, but might be crashy.  The rough sketch is here:
https://chromium-review.googlesource.com/c/chromium/src/+/1483680

Bug: 932879
Change-Id: I3b05e17fce40b9095de7eb6bc5ef64645a69a11b
Reviewed-on: https://chromium-review.googlesource.com/c/1484073Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Antoine Labour <piman@chromium.org>
Auto-Submit: Frank Liberato <liberato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635160}

content/renderer/render_thread_impl.cc

@@ -1307,6 +1307,11 @@ media::GpuVideoAcceleratorFactories* RenderThreadImpl::GetGpuFactories() {
       EstablishGpuChannelSync();
   if (!gpu_channel_host)
     return nullptr;
+  // Currently, VideoResourceUpdater can't convert hardware resources to
+  // software resources in software compositing mode.  So, fall back to software
+  // video decoding if gpu compositing is off.
+  if (is_gpu_compositing_disabled_)
+    return nullptr;
   // This context is only used to create textures and mailbox them, so
   // use lower limits than the default.
   gpu::SharedMemoryLimits limits = gpu::SharedMemoryLimits::ForMailboxContext();