Bug 827844 - Fix flickering when layout changes on Firefox/android. r=kats,bgirard

Fix flickering that can occur when the surface size changes due to a layout
change (such as the virtual keyboard appearing/disappearing) on Android.

mobile/android/base/gfx/GLController.java

@@ -74,6 +74,10 @@ public class GLController {
         notifyAll();
     }
 
+    public boolean hasValidSurface() {
+        return mSurfaceValid;
+    }
+
     private void initEGL() {
         mEGL = (EGL10)EGLContext.getEGL();