viz: Spawn renderer with gpu disabled when it's turned off already.

When the browser process sees that gpu compositing is off, it spawns
the renderer with --disable-gpu-compositing so that it does not try
to set up gpu compositing, then fall back to software, making wasted
work.

With viz this branch was being skipped since the ImageTransportFactory
was not doing anything useful yet. Now that it is, we can query it the
same way as we do in non-viz, since it watches the authoritative
compositing mode, and checks the command line on startup itself.

R=kylechar@chromium.org, piman@chromium.org

Bug: 730660
Change-Id: Ibb4a4879f0fe78fa801144f4e7af6635722529f0
Reviewed-on: https://chromium-review.googlesource.com/794910
Commit-Queue: danakj <danakj@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Reviewed-by: kylechar <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520183}

content/browser/renderer_host/render_process_host_impl.cc

@@ -2740,15 +2740,15 @@ void RenderProcessHostImpl::PropagateBrowserCommandLineToRenderer(
     renderer_cmd->AppendSwitch(switches::kDisableDatabases);
   }
 
-#if !defined(OS_ANDROID)
+#if !defined(OS_ANDROID) && !defined(OS_CHROMEOS)
   // If gpu compositing is not being used, tell the renderer at startup. This
   // is inherently racey, as it may change while the renderer is being launched,
   // but the renderer will hear about the correct state eventually. This
   // optimizes the common case to avoid wasted work.
-  // Note: There is no ImageTransportFactory with Mash or Viz.
-  // TODO(danakj): Get this info somewhere for viz mode.
-  if (!IsUsingMus() && !browser_cmd.HasSwitch(switches::kEnableViz) &&
-      ImageTransportFactory::GetInstance()->IsGpuCompositingDisabled())
+  // Note: There is no ImageTransportFactory with Mus, but there is also no
+  // software compositing on ChromeOS where Mus is used, so no need to check
+  // this state and forward it.
+  if (ImageTransportFactory::GetInstance()->IsGpuCompositingDisabled())
     renderer_cmd->AppendSwitch(switches::kDisableGpuCompositing);
 #endif