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

ui/views/test/platform_test_helper.h

Index: ui/views/test/platform_test_helper.h
diff --git a/ui/views/test/platform_test_helper.h b/ui/views/test/platform_test_helper.h
index fb3ab103da6bd0d6cf054fac5c8449950336b183..64423afc1ef32af8953a95928b7d9638f016a35f 100644
--- a/ui/views/test/platform_test_helper.h
+++ b/ui/views/test/platform_test_helper.h
@@ -10,6 +10,11 @@
 #include "base/callback_forward.h"
 #include "base/macros.h"
 
+namespace ui {
+class ContextFactory;
+class ContextFactoryPrivate;
+}
+
 namespace views {
 
 class ViewsTestHelper;
@@ -19,7 +24,7 @@ class PlatformTestHelper {
  public:
   using Factory = base::Callback<std::unique_ptr<PlatformTestHelper>(void)>;
   PlatformTestHelper() {}
-  virtual ~PlatformTestHelper() {}
+  virtual ~PlatformTestHelper();
 
   static void set_factory(const Factory& factory);
   static std::unique_ptr<PlatformTestHelper> Create();
@@ -34,6 +39,10 @@ class PlatformTestHelper {
   // Simulate an OS-level destruction of the native window held by |widget|.
   virtual void SimulateNativeDestroy(Widget* widget);
 
+  virtual void InitializeContextFactory(
+      ui::ContextFactory** factory,
+      ui::ContextFactoryPrivate** factory_private);
+
  private:
   DISALLOW_COPY_AND_ASSIGN(PlatformTestHelper);
 };