Implement pinch-zoom scaling for main-frame scrollbars and pinch-zoom overlay scrollbars.

cc/damage_tracker.cc

Index: cc/damage_tracker.cc
diff --git a/cc/damage_tracker.cc b/cc/damage_tracker.cc
index ad433e92264419aa298aec773a5de2b528f46058..99379ce3a1db6b4c3a9f6a014a812256e5f31ae4 100644
--- a/cc/damage_tracker.cc
+++ b/cc/damage_tracker.cc
@@ -162,7 +162,7 @@ gfx::RectF DamageTracker::removeRectFromCurrentFrame(int layerID, bool& layerIsN
 void DamageTracker::saveRectForNextFrame(int layerID, const gfx::RectF& targetSpaceRect)
 {
     // This layer should not yet exist in next frame's history.
-    DCHECK(layerID > 0);
+    DCHECK(layerID > 0 || layerID == -1 || layerID == -2);

[Ian Vollick, 2013/01/15 20:33:48]

I'll make a more detailed proposal about how to handle layer id's and impl-only
layers later, but if we were to go with my approach, I think DCHECK(layerID)
would be more appropriate here.

     DCHECK(m_nextRectHistory->find(layerID) == m_nextRectHistory->end());
     (*m_nextRectHistory)[layerID] = targetSpaceRect;
 }