Implement desktop compatibility viewport

cc/layer_tree_host_impl.cc

Index: cc/layer_tree_host_impl.cc
diff --git a/cc/layer_tree_host_impl.cc b/cc/layer_tree_host_impl.cc
index 9fa90e1df1a39ed29c00a87b130f75bcb3a52989..11a4979e653a53e0b4870e265161ba136b454d63 100644
--- a/cc/layer_tree_host_impl.cc
+++ b/cc/layer_tree_host_impl.cc
@@ -807,6 +807,8 @@ void LayerTreeHostImpl::drawLayers(FrameData& frame)
     if (m_activeTree->hud_layer())
         m_activeTree->hud_layer()->updateHudTexture(m_resourceProvider.get());
 
+    m_activeTree->UpdateFixedPositionViewport();
+
     m_renderer->drawFrame(frame.renderPasses);
     // The render passes should be consumed by the renderer.
     DCHECK(frame.renderPasses.empty());
@@ -1482,6 +1484,10 @@ scoped_ptr<ScrollAndScaleSet> LayerTreeHostImpl::processScrollDeltas()
     collectScrollDeltas(scrollInfo.get(), rootLayer());
     scrollInfo->pageScaleDelta = m_pinchZoomViewport.page_scale_delta();
     m_pinchZoomViewport.set_sent_page_scale_delta(scrollInfo->pageScaleDelta);
+    if (rootScrollLayer()) {
+        scrollInfo->desktopCompatibilityViewportOffsetFromScrollPositionDelta = gfx::ToFlooredVector2d(rootScrollLayer()->desktopCompatibilityViewportOffsetFromScrollPositionDelta());
+        rootScrollLayer()->setSentDesktopCompatibilityViewportOffsetFromScrollPositionDelta(scrollInfo->desktopCompatibilityViewportOffsetFromScrollPositionDelta);
+    }
 
     return scrollInfo.Pass();
 }