[Fuchsia] Fix crash in DefaultDecoderFactory

Previously DefaultDecoderFactory could crash when creating decoders
after the GPU context has been lost because it was assuming that
GetMediaContextProvider() always returns a valid result. Fixed it to it
to validate the result before trying to use it.

Bug: 1057959
Change-Id: I767a8fa7dcfcfb122be98449cc1aa46c43ceae62
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2135531
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Auto-Submit: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756374}

media/renderers/default_decoder_factory.cc

@@ -121,10 +121,22 @@ void DefaultDecoderFactory::CreateVideoDecoders(
 #if defined(OS_FUCHSIA)
   if (gpu_factories) {
     auto* context_provider = gpu_factories->GetMediaContextProvider();
-    DCHECK(context_provider);
-    video_decoders->push_back(
-        CreateFuchsiaVideoDecoder(gpu_factories->SharedImageInterface(),
-                                  context_provider->ContextSupport()));
+
+    // GetMediaContextProvider() may return nullptr when the context was lost
+    // (e.g. after GPU process crash). To handle this case RenderThreadImpl
+    // creates a new GpuVideoAcceleratorFactories with a new ContextProvider
+    // instance, but there is no way to get it here. For now just don't add
+    // FuchsiaVideoDecoder in that scenario.
+    //
+    // TODO(crbug.com/580386): Handle context loss properly.
+    if (context_provider) {
+      video_decoders->push_back(
+          CreateFuchsiaVideoDecoder(gpu_factories->SharedImageInterface(),
+                                    context_provider->ContextSupport()));
+    } else {
+      DLOG(ERROR)
+          << "Can't created FuchsiaVideoDecoder due to GPU context loss.";
+    }
   }
 
   if (base::CommandLine::ForCurrentProcess()->HasSwitch(