Make LayerTreeImpl::ScrollableArea more robust

cc/trees/layer_tree_host_unittest_scroll.cc

Index: cc/trees/layer_tree_host_unittest_scroll.cc
diff --git a/cc/trees/layer_tree_host_unittest_scroll.cc b/cc/trees/layer_tree_host_unittest_scroll.cc
index 76a369b9aa9048ec6934d9105b631acd4e17fd14..80187541c900b9f5f0a1adf5eef35b2c6d47b812 100644
--- a/cc/trees/layer_tree_host_unittest_scroll.cc
+++ b/cc/trees/layer_tree_host_unittest_scroll.cc
@@ -7,6 +7,7 @@
 #include "cc/layers/content_layer.h"
 #include "cc/layers/layer.h"
 #include "cc/layers/layer_impl.h"
+#include "cc/test/fake_content_layer.h"
 #include "cc/test/fake_content_layer_client.h"
 #include "cc/test/geometry_test_utils.h"
 #include "cc/test/layer_tree_test.h"
@@ -661,5 +662,45 @@ class LayerTreeHostScrollTestScrollZeroMaxScrollOffset
 SINGLE_AND_MULTI_THREAD_TEST_F(
     LayerTreeHostScrollTestScrollZeroMaxScrollOffset);
 
+class LayerTreeHostScrollTestScrollUpdateMaxScrollOffset
+    : public LayerTreeHostScrollTest {
+ public:
+  LayerTreeHostScrollTestScrollUpdateMaxScrollOffset() {}
+
+  virtual void BeginTest() OVERRIDE {
+    PostSetNeedsCommitToMainThread();
+  }
+
+  virtual void SetupTree() OVERRIDE {
+    LayerTreeTest::SetupTree();
+    first_content_layer_ = FakeContentLayer::Create(&client_);
+    first_content_layer_->SetBounds(gfx::Size(1, 256));
+    first_content_layer_->set_layer_animation_delegate(this);
+    layer_tree_host()->root_layer()->AddChild(first_content_layer_);
+    second_content_layer_ = FakeContentLayer::Create(&client_);
+    second_content_layer_->SetBounds(gfx::Size(256, 1));
+    second_content_layer_->set_layer_animation_delegate(this);
+    layer_tree_host()->root_layer()->AddChild(second_content_layer_);
+    layer_tree_host()->root_layer()->SetScrollable(true);
+  }
+
+  virtual void DrawLayersOnThread(LayerTreeHostImpl* impl) OVERRIDE {
+    LayerImpl* root = impl->active_tree()->root_layer();
+    EXPECT_EQ(255, root->max_scroll_offset().x());
+    EXPECT_EQ(255, root->max_scroll_offset().y());
+    EndTest();
+  }
+
+  virtual void AfterTest() OVERRIDE {}
+
+ private:
+  FakeContentLayerClient client_;
+  scoped_refptr<FakeContentLayer> first_content_layer_;
+  scoped_refptr<FakeContentLayer> second_content_layer_;
+};
+
+SINGLE_AND_MULTI_THREAD_TEST_F(
+    LayerTreeHostScrollTestScrollUpdateMaxScrollOffset);
+
 }  // namespace
 }  // namespace cc