Fix spurious 1px scrolling.

cc/trees/layer_tree_host_impl_unittest.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_host_impl.h"
 
 #include <cmath>
 
 #include "base/bind.h"
 #include "base/command_line.h"
@@ skipping 3023 matching lines @@
   host_impl_->ScrollBy(gfx::Point(), wheel_scroll_delta);
   host_impl_->ScrollEnd();
 
   // The scale should not have been applied to the scroll delta.
   scroll_info = host_impl_->ProcessScrollDeltas();
   ExpectContains(*scroll_info.get(),
                  scroll_layer->id(),
                  wheel_scroll_delta);
 }
 
+TEST_F(LayerTreeHostImplTest, ScrollViewportRounding) {
+  int width = 332;
+  int height = 20;
+  int scale = 3;
+  SetupScrollAndContentsLayers(gfx::Size(width, height));
+  host_impl_->SetViewportSize(gfx::Size(width * scale - 1, height * scale));
+  host_impl_->SetDeviceScaleFactor(scale);
+  host_impl_->active_tree()->SetPageScaleFactorAndLimits(1.f, 0.5f, 4.f);
+
+  LayerImpl* inner_viewport_scroll_layer =
+      host_impl_->active_tree()->InnerViewportScrollLayer();
+  EXPECT_EQ(gfx::Vector2d(0, 0),
+            inner_viewport_scroll_layer->MaxScrollOffset());
+}
+
 class TestScrollOffsetDelegate : public LayerScrollOffsetDelegate {
  public:
   TestScrollOffsetDelegate()
       : page_scale_factor_(0.f),
         min_page_scale_factor_(-1.f),
         max_page_scale_factor_(-1.f) {}
 
   virtual ~TestScrollOffsetDelegate() {}
 
   virtual gfx::Vector2dF GetTotalScrollOffset() OVERRIDE {
@@ skipping 3971 matching lines @@
   host_impl_->DidLoseOutputSurface();
   EXPECT_TRUE(host_impl_->IsContextLost());
   EXPECT_EQ(1, num_lost_surfaces_);
   host_impl_->DidLoseOutputSurface();
   EXPECT_TRUE(host_impl_->IsContextLost());
   EXPECT_EQ(1, num_lost_surfaces_);
 }
 
 }  // namespace
 }  // namespace cc