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

cc/layer.h

Index: cc/layer.h
diff --git a/cc/layer.h b/cc/layer.h
index 2a654d9499b5d9d28285d4e440937578a05f809e..b69681a1fd0c17c7dfb4e37ad33b19c7386a9a0a 100644
--- a/cc/layer.h
+++ b/cc/layer.h
@@ -54,6 +54,7 @@ public:
 
     static scoped_refptr<Layer> create();
 
+    static const int s_invalidLayerId;

[enne (OOO), 2013/03/11 21:08:24]

Can you make this an enum to not require storage for this value?

Also, 0 is the invalid layer id.  -1 here means something special for pinch zoom
scrollbars in that they are managed by LayerTreeImpl specially.  Can you make
them different values?

[wjmaclean, 2013/03/12 16:13:07]

Yes, good idea!
Hmm, if you refer back to comment #20, James R requested that I *don't* use 0,
on the grounds that -1 is the invalid layer id (not the exact same context, but
still we're marking invalid layers). I can live with either, but it would be
good if there could be a common consensus on which value to use.

I have proposed the following solution in the revised patch:

-1 => invalid layer
0 => unused layer (PZ scrollbar layers use this for scrolling layer id)

     int id() const;
 
     Layer* rootLayer();