views/mus: Install a mus-friend ContextFactory for tests.

cc/test/fake_compositor_frame_sink.cc

Index: cc/test/fake_compositor_frame_sink.cc
diff --git a/cc/test/fake_compositor_frame_sink.cc b/cc/test/fake_compositor_frame_sink.cc
index a4e19cb09524645460ddbb38a17323916769b298..ae75f85316a8c8060345bd23544eb5d3b20a2f24 100644
--- a/cc/test/fake_compositor_frame_sink.cc
+++ b/cc/test/fake_compositor_frame_sink.cc
@@ -8,6 +8,8 @@
 #include "base/threading/thread_task_runner_handle.h"
 #include "cc/output/compositor_frame_sink_client.h"
 #include "cc/resources/returned_resource.h"
+#include "cc/scheduler/begin_frame_source.h"
+#include "cc/scheduler/delay_based_time_source.h"
 #include "cc/test/begin_frame_args_test.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
@@ -29,6 +31,16 @@ FakeCompositorFrameSink::FakeCompositorFrameSink(
 
 FakeCompositorFrameSink::~FakeCompositorFrameSink() = default;
 
+bool FakeCompositorFrameSink::BindToClient(CompositorFrameSinkClient* client) {
+  if (!CompositorFrameSink::BindToClient(client))
+    return false;
+  begin_frame_source_ = base::MakeUnique<BackToBackBeginFrameSource>(
+      base::MakeUnique<DelayBasedTimeSource>(
+          base::ThreadTaskRunnerHandle::Get().get()));
+  client_->SetBeginFrameSource(begin_frame_source_.get());
+  return true;
+}
+
 void FakeCompositorFrameSink::DetachFromClient() {
   ReturnResourcesHeldByParent();
   CompositorFrameSink::DetachFromClient();