Wait on frame work fences when frames are dropped

bug:30895941

Prevents a race where frame work could interleave between frames,
causing SurfaceView position updates to be delivered out of order.

Change-Id: I01e4cc557b69dcf33e877a0e16c0d115ec95e4cc
diff --git a/libs/hwui/renderthread/CanvasContext.h b/libs/hwui/renderthread/CanvasContext.h
index 3eef29b..e182175 100644
--- a/libs/hwui/renderthread/CanvasContext.h
+++ b/libs/hwui/renderthread/CanvasContext.h
@@ -168,6 +168,8 @@
 
     ANDROID_API int64_t getFrameNumber();
 
+    void waitOnFences();
+
 private:
     friend class RegisterFrameCallbackTask;
     // TODO: Replace with something better for layer & other GL object
@@ -178,8 +180,6 @@
 
     void freePrefetchedLayers(TreeObserver* observer);
 
-    void waitOnFences();
-
     bool isSwapChainStuffed();
 
     EGLint mLastFrameWidth = 0;