Hide pinch viewport scrollbars when near minimum scale.

cc/trees/layer_tree_impl.cc

Index: cc/trees/layer_tree_impl.cc
diff --git a/cc/trees/layer_tree_impl.cc b/cc/trees/layer_tree_impl.cc
index 041c5fea5186105c11c86371cea45ed66c3ee206..9cda9535a277934d99fe0e3cdf9993515bb1c5cd 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -395,6 +395,29 @@ void LayerTreeImpl::DidUpdatePageScale() {
   }
 
   ForceScrollbarParameterUpdateAfterScaleChange(page_scale_layer());
+
+  HideInnerViewportScrollbarsIfNearMinimumScale();
+}
+
+void LayerTreeImpl::HideInnerViewportScrollbarsIfNearMinimumScale() {
+  if (!InnerViewportContainerLayer())
+    return;
+
+  LayerImpl::ScrollbarSet* scrollbars =
+      InnerViewportContainerLayer()->scrollbars();
+
+  if (!scrollbars)
+    return;
+
+  for (LayerImpl::ScrollbarSet::iterator it = scrollbars->begin();
+       it != scrollbars->end();
+       ++it) {
+    ScrollbarLayerImplBase* scrollbar = *it;
+    float minimum_scale_to_show_at =
+        min_page_scale_factor() + settings().scrollbar_show_scale_threshold;

[aelias_OOO_until_Jul13, 2015/01/27 19:35:05]

I generally prefer scales to be multiplied together rather than added, adding
can have strange nonlinear effects.  Please make this a multiplication.

[bokan, 2015/01/27 20:42:01]

Done.

+    scrollbar->SetHideLayerAndSubtree(
+        current_page_scale_factor() < minimum_scale_to_show_at);
+  }
 }
 
 SyncedProperty<ScaleGroup>* LayerTreeImpl::page_scale_factor() {
@@ -476,6 +499,8 @@ void LayerTreeImpl::SetViewportLayersFromIds(
   DCHECK(outer_viewport_scroll_layer_ ||
          outer_viewport_scroll_layer_id == Layer::INVALID_ID);
 
+  HideInnerViewportScrollbarsIfNearMinimumScale();
+
   if (!root_layer_scroll_offset_delegate_)
     return;