Implement RenderWidgetHostViewAndroid::GetLastScrollOffset

Apparently this code was being hit on Android. This CL implements the scroll
offset the same way it's implemented on other platforms.

BUG=409792
NOTRY=true
TBR=aelias@chromium.org

Review URL: https://codereview.chromium.org/533483004

Cr-Commit-Position: refs/heads/master@{#292956}

content/browser/renderer_host/render_widget_host_view_android.cc

@@ -427,8 +427,7 @@ bool RenderWidgetHostViewAndroid::HasValidFrame() const {
 }
 
 gfx::Vector2dF RenderWidgetHostViewAndroid::GetLastScrollOffset() const {
-  NOTREACHED();
-  return gfx::Vector2dF();
+  return last_scroll_offset_;
 }
 
 gfx::NativeView RenderWidgetHostViewAndroid::GetNativeView() const {
@@ -1034,6 +1033,7 @@ void RenderWidgetHostViewAndroid::ComputeContentsSize(
 void RenderWidgetHostViewAndroid::InternalSwapCompositorFrame(
     uint32 output_surface_id,
     scoped_ptr<cc::CompositorFrame> frame) {
+  last_scroll_offset_ = frame->metadata.root_scroll_offset;
   if (!frame->delegated_frame_data) {
     LOG(ERROR) << "Non-delegated renderer path no longer supported";
     return;

content/browser/renderer_host/render_widget_host_view_android.h

@@ -429,6 +429,9 @@ class CONTENT_EXPORT RenderWidgetHostViewAndroid
   // List of readbackrequests waiting for arrival of a valid frame.
   std::queue<ReadbackRequest> readbacks_waiting_for_frame_;
 
+  // The last scroll offset of the view.
+  gfx::Vector2dF last_scroll_offset_;
+
   DISALLOW_COPY_AND_ASSIGN(RenderWidgetHostViewAndroid);
 };