Continue threading context through unit tests.

ash/test/ash_test_base.h

Index: ash/test/ash_test_base.h
diff --git a/ash/test/ash_test_base.h b/ash/test/ash_test_base.h
index 7a1c23bca1236c098b05eec1cfdd91554969f4da..40ae938c38607cf3f0490631efd3e47f9cce8694 100644
--- a/ash/test/ash_test_base.h
+++ b/ash/test/ash_test_base.h
@@ -56,6 +56,11 @@ class AshTestBase : public testing::Test {
   // See ash::test::DisplayManagerTestApi::UpdateDisplay for more details.
   void UpdateDisplay(const std::string& display_specs);
 
+  // Returns a RootWindow. Usually this is the active RootWindow, but that
+  // method can return NULL sometimes, and in those cases, we fall back on the
+  // primary RootWindow.
+  aura::RootWindow* CurrentContext();
+
   // Versions of the functions in aura::test:: that go through our shell
   // StackingController instead of taking a parent.
   aura::Window* CreateTestWindowInShellWithId(int id);