mus/viz: Renderer needs to know the real viz host.

Renderer needs to talk to the browser if browser is hosting viz, even
when running with mus. So make sure renderer talks to the right process
for viz-related communication.

BUG=786453

Change-Id: I3561d9f1d8ae14bc35a34125fd55df773afeeca1
Reviewed-on: https://chromium-review.googlesource.com/777439
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Fady Samuel <fsamuel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517704}

content/renderer/render_thread_impl.cc

@@ -333,6 +333,16 @@ void AddHistogramSample(void* hist, int sample) {
   histogram->Add(sample);
 }
 
+bool IsMusHostingViz() {
+#if BUILDFLAG(ENABLE_MUS)
+  const auto* cmdline = base::CommandLine::ForCurrentProcess();
+  return cmdline->GetSwitchValueASCII(switches::kMus) ==
+         switches::kMusHostVizValue;
+#else
+  return false;
+#endif
+}
+
 class FrameFactoryImpl : public mojom::FrameFactory {
  public:
   explicit FrameFactoryImpl(const service_manager::BindSourceInfo& source_info)
@@ -690,7 +700,7 @@ void RenderThreadImpl::Init(
 
   gpu_ = ui::Gpu::Create(
       GetConnector(),
-      IsRunningWithMus() ? ui::mojom::kServiceName : mojom::kBrowserServiceName,
+      IsMusHostingViz() ? ui::mojom::kServiceName : mojom::kBrowserServiceName,
       GetIOTaskRunner());
 
   viz::mojom::SharedBitmapAllocationNotifierPtr
@@ -2043,7 +2053,7 @@ void RenderThreadImpl::RequestNewLayerTreeFrameSink(
   }
 
 #if defined(USE_AURA)
-  if (IsRunningWithMus()) {
+  if (IsMusHostingViz()) {
     if (!RendererWindowTreeClient::Get(routing_id)) {
       callback.Run(nullptr);
       return;