Bug 748382. Don't count overscroll as checkerboard. r=kats

We currently check root layer position with the viewport.
During overscroll these won't match and we'll think we're
checkerboarding.

--HG--
extra : rebase_source : 27c93b524a539ad5851ff4ba5ee94f5321c24172

mobile/android/base/gfx/LayerRenderer.java

@@ -639,6 +639,15 @@ public class LayerRenderer implements GLSurfaceView.Renderer {
                 // Find out how much of the viewport area is valid
                 Rect viewport = RectUtils.round(mPageContext.viewport);
                 Region validRegion = rootLayer.getValidRegion(mPageContext);
+
+                /* restrict the viewport to page bounds so we don't
+                 * count overscroll as checkerboard */
+                if (!viewport.intersect(mPageRect)) {
+                    /* if the rectangles don't intersect
+                       intersect() doesn't change viewport
+                       so we set it to empty by hand */
+                    viewport.setEmpty();
+                }
                 validRegion.op(viewport, Region.Op.INTERSECT);
 
                 float checkerboard = 0.0f;