Fix main thread top controls scrolling to mirror CC.

content/renderer/render_view_impl.cc

Index: content/renderer/render_view_impl.cc
diff --git a/content/renderer/render_view_impl.cc b/content/renderer/render_view_impl.cc
index 75cc2fb5cbe3015ef1a4ecd215cde43cae31906a..b2557de9c920555438dbdf601ecd1b8b62091876 100644
--- a/content/renderer/render_view_impl.cc
+++ b/content/renderer/render_view_impl.cc
@@ -627,6 +627,8 @@ RenderViewImpl::RenderViewImpl(CompositorDependencies* compositor_deps,
 #if defined(OS_ANDROID)
       top_controls_constraints_(TOP_CONTROLS_STATE_BOTH),
 #endif
+      top_controls_shrink_blink_size_(false),
+      top_controls_height_(0.f),
       has_focus_(false),
       has_scrolled_focused_editable_node_into_rect_(false),
       main_render_frame_(nullptr),
@@ -2615,6 +2617,22 @@ void RenderViewImpl::OnResize(const ResizeParams& params) {
 
   gfx::Size old_visible_viewport_size = visible_viewport_size_;
 
+  if (top_controls_height_ || params.top_controls_height) {
+    // When top controls are present, we need to resize the Blink widget and
+    // change the top controls height at the same time to prevent the viewport
+    // from changing scroll offset due to a clamp in between the two operations.
+    top_controls_shrink_blink_size_ = params.top_controls_shrink_blink_size;
+    top_controls_height_ = params.top_controls_height;
+    gfx::Size new_view_size = params.new_size;
+    if (IsUseZoomForDSFEnabled()) {
+      new_view_size = gfx::ScaleToCeiledSize(new_view_size,
+                                             GetOriginalDeviceScaleFactor());
+    }
+    webview()->resizeWithTopControls(new_view_size,

[no sievers, 2016/04/08 19:26:02]

webview() is null-checked everywhere else in OnResize() here and in
RenderWidget::OnResize() also.

[bokan, 2016/04/11 19:54:15]

This is gone now in addressing the comment below.

+                                     top_controls_height_,

[no sievers, 2016/04/08 19:26:02]

I find it slightly weird that we end up calling resize() twice effectively here,
even if it doesn't do anything the second time.

In fact, I'm wondering if it can cause subtle bugs because when we go through
the rest of the code there is a certain order assumed for things to happen, but
here for Android we now actually resize the widget first.

When exactly (or from which call site) do you want this not to happen i.e. why
did you have to move it?

[bokan, 2016/04/11 19:54:15]

I moved it since we didn't want this method to exist on WebWidget since it makes
no sense there. Thus, we need access to the WebView rather than WebWidget.

You're right though we're subtly changing the order here. I've rearranged this
so that RenderViewImpl overrides a ResizeWebWidget method that gets called from
RenderWidget::Resize in the same place as before. This should be less of a
change.

+                                     top_controls_shrink_blink_size_);
+  }
+
   RenderWidget::OnResize(params);
 
   if (old_visible_viewport_size != visible_viewport_size_)