Route Scroll events through a cc::InputHandler

ui/compositor/compositor.cc

Index: ui/compositor/compositor.cc
diff --git a/ui/compositor/compositor.cc b/ui/compositor/compositor.cc
index 63f72bc6178b815d9008fb0daff580c3ae1db9fd..30013a97dfc771261bd2c0fe115762d75537368e 100644
--- a/ui/compositor/compositor.cc
+++ b/ui/compositor/compositor.cc
@@ -39,6 +39,7 @@
 #include "ui/compositor/dip_util.h"
 #include "ui/compositor/layer.h"
 #include "ui/compositor/layer_animator_collection.h"
+#include "ui/compositor/overscroll/ui_scroll_input_manager.h"
 #include "ui/gl/gl_switches.h"
 
 namespace {
@@ -183,6 +184,10 @@ Compositor::Compositor(ui::ContextFactory* context_factory,
   settings.gpu_memory_policy.priority_cutoff_when_visible =
       gpu::MemoryAllocation::CUTOFF_ALLOW_NICE_TO_HAVE;
 
+#if defined(OS_MACOSX)
+  settings.enable_elastic_overscroll = true;
+#endif
+
   base::TimeTicks before_create = base::TimeTicks::Now();
 
   animation_host_ = cc::AnimationHost::CreateMainInstance();
@@ -194,6 +199,14 @@ Compositor::Compositor(ui::ContextFactory* context_factory,
   params.main_task_runner = task_runner_;
   params.mutator_host = animation_host_.get();
   host_ = cc::LayerTreeHostInProcess::CreateSingleThreaded(this, &params);
+
+  // Tie composited scrolling with whether the platform wants elastic scrolling.
+  // TODO(tapted): Use composited scrolling on all platforms.
+  if (settings.enable_elastic_overscroll) {
+    scroll_input_manager_.reset(
+        new UIScrollInputManager(host_->GetInputHandler()));
+  }
+
   UMA_HISTOGRAM_TIMES("GPU.CreateBrowserCompositor",
                       base::TimeTicks::Now() - before_create);