Add viewport scrollbar class to support overlay scrollbars for pinch zoom virtual viewport.

cc/trees/layer_tree_host.cc

Index: cc/trees/layer_tree_host.cc
diff --git a/cc/trees/layer_tree_host.cc b/cc/trees/layer_tree_host.cc
index 5e19eb3eb3c8acb70f68dba1d64863f6bb806297..bb81ee8a4497a084e8c2f91df0286007db1512a9 100644
--- a/cc/trees/layer_tree_host.cc
+++ b/cc/trees/layer_tree_host.cc
@@ -20,6 +20,7 @@
 #include "cc/base/thread.h"
 #include "cc/debug/overdraw_metrics.h"
 #include "cc/debug/rendering_stats_instrumentation.h"
+#include "cc/input/inner_viewport_scrollbar.h"
 #include "cc/input/top_controls_manager.h"
 #include "cc/layers/heads_up_display_layer.h"
 #include "cc/layers/heads_up_display_layer_impl.h"
@@ -314,6 +315,14 @@ void LayerTreeHost::FinishCommitOnImplThread(LayerTreeHostImpl* host_impl) {
 
   sync_tree->FindRootScrollLayer();
 
+  if (settings_.use_pinch_virtual_viewport && page_scale_layer_) {
+    DCHECK(inner_viewport_scroll_layer_);
+    DCHECK(outer_viewport_scroll_layer_);
+    sync_tree->SetPinchViewportLayerIds(page_scale_layer_->id(),
+                                        inner_viewport_scroll_layer_->id(),
+                                        outer_viewport_scroll_layer_->id());
+  }
+
   float page_scale_delta, sent_page_scale_delta;
   if (settings_.impl_side_painting) {
     // Update the delta from the active tree, which may have
@@ -544,6 +553,58 @@ void LayerTreeHost::SetRootLayer(scoped_refptr<Layer> root_layer) {
   SetNeedsFullTreeSync();
 }
 
+static void SetupInnerViewportScrollbar(Layer* parent,
+                                        scoped_refptr<Layer>& scrollbar,
+                                        scoped_refptr<Layer> scroll_layer,
+                                        scoped_refptr<Layer> clip_layer,
+                                        ScrollbarOrientation orientation) {
+  int scrollbar_thickness =
+      orientation == HORIZONTAL ? parent->bounds().height()
+                                : parent->bounds().width();
+  scrollbar = ScrollbarLayer::Create(
+      make_scoped_ptr(new InnerViewportScrollbar(
+          scroll_layer, clip_layer, orientation, scrollbar_thickness))
+          .PassAs<Scrollbar>(),
+      scroll_layer->id());
+  parent->RemoveAllChildren();
+  parent->AddChild(scrollbar);
+  scrollbar->SetBounds(parent->bounds());
+  scrollbar->SetIsDrawable(true);
+}
+
+void LayerTreeHost::RegisterPinchViewportLayers(
+      scoped_refptr<Layer> inner_viewport_clip_layer,
+      scoped_refptr<Layer> page_scale_layer,
+      scoped_refptr<Layer> inner_viewport_scroll_layer,
+      scoped_refptr<Layer> outer_viewport_scroll_layer,
+      scoped_refptr<Layer> inner_viewport_horizontal_scrollbar_layer,
+      scoped_refptr<Layer> inner_viewport_vertical_scrollbar_layer) {
+  inner_viewport_clip_layer_ = inner_viewport_clip_layer;
+  page_scale_layer_ = page_scale_layer;
+  inner_viewport_scroll_layer_ = inner_viewport_scroll_layer;
+  outer_viewport_scroll_layer_ = outer_viewport_scroll_layer;
+
+  if (inner_viewport_horizontal_scrollbar_layer) {
+    SetupInnerViewportScrollbar(inner_viewport_horizontal_scrollbar_layer,
+        inner_viewport_horizontal_scrollbar_layer_,
+        inner_viewport_scroll_layer,
+        inner_viewport_clip_layer,
+        HORIZONTAL);
+  } else {
+    inner_viewport_horizontal_scrollbar_layer_ = nullptr;
+  }
+
+  if (inner_viewport_vertical_scrollbar_layer) {
+    SetupInnerViewportScrollbar(inner_viewport_vertical_scrollbar_layer,
+        inner_viewport_vertical_scrollbar_layer_,
+        inner_viewport_scroll_layer,
+        inner_viewport_clip_layer,
+        VERTICAL);
+  } else {
+    inner_viewport_vertical_scrollbar_layer_ = nullptr;
+  }
+}
+
 void LayerTreeHost::SetDebugState(const LayerTreeDebugState& debug_state) {
   LayerTreeDebugState new_debug_state =
       LayerTreeDebugState::Unite(settings_.initial_debug_state, debug_state);
@@ -717,6 +778,8 @@ void LayerTreeHost::UpdateLayers(Layer* root_layer,
     UpdateHudLayer();
 
     Layer* root_scroll = FindFirstScrollableLayer(root_layer);
+    Layer* page_scale_layer =
+        page_scale_layer_ ? page_scale_layer_.get() : root_scroll;
 
     TRACE_EVENT0("cc", "LayerTreeHost::UpdateLayers::CalcDrawProps");
     LayerTreeHostCommon::CalculateDrawProperties(
@@ -725,7 +788,7 @@ void LayerTreeHost::UpdateLayers(Layer* root_layer,
         gfx::Transform(),
         device_scale_factor_,
         page_scale_factor_,
-        root_scroll,
+        page_scale_layer,
         GetRendererCapabilities().max_texture_size,
         settings_.can_use_lcd_text,
         settings_.layer_transforms_should_scale_layer_contents,