Mark GrContext dirty after onContextDestroyed

Bug: 120997728
Test: hwui_unit_test passes
Change-Id: I8b60c45ebf7904b7026b480541dac78b70a59e0d
diff --git a/libs/hwui/renderthread/RenderThread.h b/libs/hwui/renderthread/RenderThread.h
index 12666b3..5272227 100644
--- a/libs/hwui/renderthread/RenderThread.h
+++ b/libs/hwui/renderthread/RenderThread.h
@@ -123,6 +123,7 @@
     friend class RenderProxy;
     friend class DummyVsyncSource;
     friend class android::uirenderer::TestUtils;
+    friend class android::uirenderer::WebViewFunctor;
 
     RenderThread();
     virtual ~RenderThread();