[Fuchsia] Disable SW video decoders in isolated cast_runner context

Software video decoders were disabled in the default context used in
cast_runner, but not in the isolated context. It should be disabled
for all context instances used in cast.

Bug: 1035226
Change-Id: I5c17196e590a07b4e9557f353912786fcd04c9b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1971052
Auto-Submit: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#726521}

fuchsia/runners/cast/cast_runner.cc

@@ -35,7 +35,8 @@ fuchsia::web::CreateContextParams BuildCreateContextParamsForIsolatedRunners(
   output.set_features(
       fuchsia::web::ContextFeatureFlags::AUDIO |
       fuchsia::web::ContextFeatureFlags::VULKAN |
-      fuchsia::web::ContextFeatureFlags::HARDWARE_VIDEO_DECODER);
+      fuchsia::web::ContextFeatureFlags::HARDWARE_VIDEO_DECODER |
+      fuchsia::web::ContextFeatureFlags::HARDWARE_VIDEO_DECODER_ONLY);
 
   if (create_context_params.has_user_agent_product()) {
     output.set_user_agent_product(create_context_params.user_agent_product());