[HWUI] Fix invisible views are still shown in popup window

Symptom: Invisible views are still shown in popup window

Root Cause: glClear isn't called if there is no draw command

Solution: Even if there is no draw command, it still calls glClear
          by startFrame().

Reproduce steps: App set one view in a popup window to invisible,
                 it will be still visible.

Change-Id: I36bc7b752434ad388fbb80ea63341ac778f48d35
diff --git a/libs/hwui/OpenGLRenderer.cpp b/libs/hwui/OpenGLRenderer.cpp
old mode 100644
new mode 100755
index 4d76bed..f0adea9
--- a/libs/hwui/OpenGLRenderer.cpp
+++ b/libs/hwui/OpenGLRenderer.cpp
@@ -2057,7 +2057,9 @@
         return status | deferredList.flush(*this, dirty);
     }
 
-    return DrawGlInfo::kStatusDone;
+    // Even if there is no drawing command(Ex: invisible),
+    // it still needs startFrame to clear buffer and start tiling.
+    return startFrame();
 }
 
 void OpenGLRenderer::outputDisplayList(DisplayList* displayList) {