Add a BrowserViewRenderer unittest for compositor switch logic

android_webview/browser/browser_view_renderer.h

Index: android_webview/browser/browser_view_renderer.h
diff --git a/android_webview/browser/browser_view_renderer.h b/android_webview/browser/browser_view_renderer.h
index 0a46c0bd5aacc25d44779b3f2c0d3ac66f8b3a44..ac70535f4bef0c4c4dc983843b06608f4910b8c7 100644
--- a/android_webview/browser/browser_view_renderer.h
+++ b/android_webview/browser/browser_view_renderer.h
@@ -138,6 +138,11 @@ class BrowserViewRenderer : public content::SynchronousCompositorClient,
 
   void SetActiveCompositorID(const CompositorID& compositor_id);
 
+  // Visible for testing.
+  content::SynchronousCompositor* GetActiveCompositorForTesting() const {
+    return compositor_;
+  }
+
  private:
   void SetTotalRootLayerScrollOffset(const gfx::Vector2dF& new_value_dip);
   bool CanOnDraw();