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

cc/layer_tree_settings.cc

Index: cc/layer_tree_settings.cc
diff --git a/cc/layer_tree_settings.cc b/cc/layer_tree_settings.cc
index b2af0bde419de74c431d7e1709ea010f28d419bd..186d0ae9ff7050215986ccd65407ad23dec7d91b 100644
--- a/cc/layer_tree_settings.cc
+++ b/cc/layer_tree_settings.cc
@@ -45,6 +45,7 @@ LayerTreeSettings::LayerTreeSettings()
     , defaultTileSize(gfx::Size(256, 256))
     , maxUntiledLayerSize(gfx::Size(512, 512))
     , minimumOcclusionTrackingSize(gfx::Size(160, 160))
+    , usePinchZoomScrollbars(true)
 {
     // TODO(danakj): Renable surface caching when we can do it more realiably. crbug.com/170713
     cacheRenderPassContents = false;