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

cc/input/inner_viewport_scrollbar.cc

+// Copyright 2013 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/input/inner_viewport_scrollbar.h"
+
+#include "cc/layers/layer.h"
+#include "third_party/skia/include/core/SkCanvas.h"
+
+namespace cc {
+
+InnerViewportScrollbar::InnerViewportScrollbar(
+    scoped_refptr<Layer> scroll_layer,
+    scoped_refptr<Layer> clip_layer,
+    ScrollbarOrientation orientation,
+    size_t thickness)
+    : scroll_layer_(scroll_layer),
+      clip_layer_(clip_layer),
+      orientation_(orientation),
+      thickness_(thickness) {}
+
+InnerViewportScrollbar::~InnerViewportScrollbar() {}
+
+ScrollbarOrientation InnerViewportScrollbar::Orientation() const {
+  return orientation_;
+}
+
+gfx::Point InnerViewportScrollbar::Location() const {
+  return gfx::Point();
+}
+
+bool InnerViewportScrollbar::IsOverlay() const {
+  return true;
+}
+
+bool InnerViewportScrollbar::HasThumb() const {
+  return true;
+}
+
+int InnerViewportScrollbar::ThumbThickness() const {
+  return thickness_;
+}
+
+int InnerViewportScrollbar::ThumbLength() const {
+  gfx::Rect track_rect = TrackRect();

[jamesr, 2013/07/03 21:12:14]

this is dead code

+  if (orientation_ == HORIZONTAL) {
+    float ratio = static_cast<float>(track_rect.width()) /
+                  clip_layer_->bounds().width();
+    return track_rect.width() - ratio * scroll_layer_->max_scroll_offset().x();

[enne (OOO), 2013/06/19 23:38:28]

(1) I think this math is not quite right.  Can you add a unit test?

In particular, if you apply a page scale, shouldn't the thumb length change? 
When I suggested using clip/scroll layer ids directly as a way to figure this
out, more specfiically I was suggesting to use the screen space clip rect and
screen space scroll layer relative to each other and not just their layer space
bounds.

(2) More importantly, why is this math on the main thread? These values will be
frozen and pushed to the ScrollbarLayerImpl, which will then not update as the
clip and scroll layers move around.

[trchen, 2013/06/20 01:16:08]

Your relative layer space approach should work with regular scrollbars too. For
the long term, do we still need two implementation of Scrollbar at all?

[wjmaclean, 2013/06/21 18:20:53]

We still need something that will work without a backing WebCore::Scrollbar,
which cc::Scrollbar still requires.

This class is not expected to ever work with non-overlay/non-innerviewport
scrollbars.

[wjmaclean, 2013/06/21 18:20:53]

Yes, I'll add a unit test.
Yes, you're right ... I'll adjust this so the thumb takes this into account.
I guess we could just provide dummy values here, and only go with the impl side
values. (I had thought we would want to have valid  'initial' values, but we
could likely do without.)

[trchen, 2013/06/24 21:38:04]

I think we can do without dummy values. Most of the time dummy accessors are
only added to make LayerTreeHostCommon templates happy. (e.g. scrollDelta)

How are you going to implement this though? Likely you'll need to update the
LayerImpl raw pointers or layer IDs during tree synchronization. Are you going
to make a twin class for the main-thread counterpart?

+  } else {
+    float ratio = static_cast<float>(track_rect.height()) /
+                  clip_layer_->bounds().height();
+    return track_rect.height() - ratio * scroll_layer_->max_scroll_offset().y();
+  }
+}
+
+gfx::Rect InnerViewportScrollbar::TrackRect() const {
+  if (orientation_ == HORIZONTAL)

[enne (OOO), 2013/06/19 23:38:28]

style nit: {}

[wjmaclean, 2013/06/21 18:20:53]

Done.

[jamesr, 2013/07/03 21:12:14]

I'm pretty sure this is completely dead code.

+    return gfx::Rect(
+        0, 0, clip_layer_->bounds().width() - thickness_, thickness_);
+  else
+    return gfx::Rect(
+        0, 0, thickness_, clip_layer_->bounds().height() - thickness_);
+}
+
+void InnerViewportScrollbar::PaintPart(SkCanvas* canvas,
+                                       ScrollbarPart part,
+                                       gfx::Rect content_rect) {}
+
+}  // namespace cc