[android_webview] Use a fraction to calculate scroll offset.

cc/trees/layer_tree_impl.cc

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/trees/layer_tree_impl.h"
 
 #include "base/debug/trace_event.h"
 #include "cc/animation/keyframed_animation_curve.h"
 #include "cc/animation/scrollbar_animation_controller.h"
 #include "cc/debug/traced_value.h"
@@ skipping 171 matching lines @@
 }
 
 void LayerTreeImpl::SetPageScaleFactorAndLimits(float page_scale_factor,
     float min_page_scale_factor, float max_page_scale_factor) {
   if (!page_scale_factor)
     return;
 
   min_page_scale_factor_ = min_page_scale_factor;
   max_page_scale_factor_ = max_page_scale_factor;
   page_scale_factor_ = page_scale_factor;
+
+  if (root_layer_scroll_offset_delegate_)
+    root_layer_scroll_offset_delegate_->SetPageScaleFactor(page_scale_factor_);
 }
 
 void LayerTreeImpl::SetPageScaleDelta(float delta) {
   // Clamp to the current min/max limits.
   float total = page_scale_factor_ * delta;
   if (min_page_scale_factor_ && total < min_page_scale_factor_)
     delta = min_page_scale_factor_ / page_scale_factor_;
   else if (max_page_scale_factor_ && total > max_page_scale_factor_)
     delta = max_page_scale_factor_ / page_scale_factor_;
 
@@ skipping 391 matching lines @@
     render_surface_layer_list->Append(TracedValue::CreateIDRef(*it).release());
   }
 
   state->Set("render_surface_layer_list",
              render_surface_layer_list.release());
   return state.PassAs<base::Value>();
 }
 
 void LayerTreeImpl::SetRootLayerScrollOffsetDelegate(
     LayerScrollOffsetDelegate* root_layer_scroll_offset_delegate) {
+  if (root_layer_scroll_offset_delegate_ == root_layer_scroll_offset_delegate)
+    return;
+
   root_layer_scroll_offset_delegate_ = root_layer_scroll_offset_delegate;
+
   if (root_scroll_layer_) {
     root_scroll_layer_->SetScrollOffsetDelegate(
         root_layer_scroll_offset_delegate_);
   }
+
+  if (root_layer_scroll_offset_delegate_) {
+    root_layer_scroll_offset_delegate_->SetScrollableSize(ScrollableSize());
+    root_layer_scroll_offset_delegate_->SetPageScaleFactor(page_scale_factor_);
+  }
 }
 
 void LayerTreeImpl::UpdateRootScrollLayerSizeDelta() {
   LayerImpl* root_scroll = RootScrollLayer();
   LayerImpl* root_container = RootContainerLayer();
   DCHECK(root_scroll);
   DCHECK(root_container);
   DCHECK(IsActiveTree());
 
   gfx::Vector2dF scrollable_viewport_size =
@@ skipping 85 matching lines @@
 const std::vector<LayerImpl*> LayerTreeImpl::LayersWithCopyOutputRequest()
     const {
   // Only the active tree needs to know about layers with copy requests, as
   // they are aborted if not serviced during draw.
   DCHECK(IsActiveTree());
 
   return layers_with_copy_output_request_;
 }
 
 }  // namespace cc