Bug 726817 - Use nsIFrameLoaderOwner.clampScrollPosition to fix clip on zoom.

Zooming in caused the right and bottom edges of the page to be clipped. This
was because we would try to scroll to coordinates that, untransformed, would be
invalid. The document has no knowledge of the zoom, and so the scroll position
needs to be forced somehow.

Java compositor accomplished this using a CSS translation transformation, this
accomplishes it by turning off scroll position clamping (a technique that the
Java compositor should also employ, if the patch this relies on passes review).

mobile/android/chrome/content/browser.js

@@ -1476,6 +1476,7 @@ Tab.prototype = {
     let frameLoader = this.browser.QueryInterface(Ci.nsIFrameLoaderOwner).frameLoader;
     if (kUsingGLLayers) {
         frameLoader.renderMode = Ci.nsIFrameLoader.RENDER_MODE_ASYNC_SCROLL;
+        frameLoader.clampScrollPosition = false;
     } else {
         // Turn off clipping so we can buffer areas outside of the browser element.
         frameLoader.clipSubdocument = false;
@@ -1593,11 +1594,6 @@ Tab.prototype = {
     this.userScrollPos.x = win.scrollX;
     this.userScrollPos.y = win.scrollY;
 
-    // If we've been asked to over-scroll, do it via the transformation
-    // and store it separately to the viewport.
-    let excessX = aViewport.x - win.scrollX;
-    let excessY = aViewport.y - win.scrollY;
-
     this._viewport.width = gScreenWidth = aViewport.width;
     this._viewport.height = gScreenHeight = aViewport.height;
     dump("### gScreenWidth = " + gScreenWidth + "\n");
@@ -1606,15 +1602,11 @@ Tab.prototype = {
     let cwu = window.top.QueryInterface(Ci.nsIInterfaceRequestor)
                          .getInterface(Ci.nsIDOMWindowUtils);
 
-    if ((aViewport.offsetX != this._viewport.offsetX) ||
-        (excessX != this.viewportExcess.x)) {
+    if (aViewport.offsetX != this._viewport.offsetX) {
       this._viewport.offsetX = aViewport.offsetX;
-      this.viewportExcess.x = excessX;
     }
-    if ((aViewport.offsetY != this._viewport.offsetY) ||
-        (excessY != this.viewportExcess.y)) {
+    if (aViewport.offsetY != this._viewport.offsetY) {
       this._viewport.offsetY = aViewport.offsetY;
-      this.viewportExcess.y = excessY;
     }
     if (Math.abs(zoom - this._viewport.zoom) >= 1e-6) {
       this._viewport.zoom = zoom;