viz: Add comment explaining GetFrameSinkManager().

VizProcessTransportFactory::GetFrameSinkManager() has a NOTREACHED()
that is intentional. This is likely to be where other teams see crashes
originating from in viz_content_browsertests, so add a better comment to
give some context for why it's happening.

Bug: none
Change-Id: Ib4eb4ff9d71a86d04359d6d20a30f6c819e49126
Reviewed-on: https://chromium-review.googlesource.com/824985Reviewed-by: James MacLean <wjmaclean@chromium.org>
Commit-Queue: kylechar <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523979}

content/browser/compositor/viz_process_transport_factory.cc

@@ -305,7 +305,11 @@ void VizProcessTransportFactory::SetOutputIsSecure(ui::Compositor* compositor,
 }
 
 viz::FrameSinkManagerImpl* VizProcessTransportFactory::GetFrameSinkManager() {
-  // FrameSinkManagerImpl is in the gpu process, not the browser process.
+  // When running with viz there is no FrameSinkManagerImpl in the browser
+  // process. FrameSinkManagerImpl runs in the GPU process instead. Anything in
+  // the browser process that relies FrameSinkManagerImpl or SurfaceManager
+  // internal state needs to change. See https://crbug.com/787097 and
+  // https://crbug.com/760181 for more context.
   NOTREACHED();
   return nullptr;
 }