Null out context_factory in window_tree_host on shutdown

A context factory is specific to a particular native viewport so we
shouldn't leak it between different WindowTreeHostImpl instances. This
fixes some crashes in mojo_view_manager_unittests.

R=sky@chromium.org

Review URL: https://codereview.chromium.org/494883002

Cr-Commit-Position: refs/heads/master@{#291196}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@291196 0039d316-1c4b-4281-b951-d872f2087c98

mojo/services/view_manager/window_tree_host_impl.cc

@@ -89,6 +89,8 @@ WindowTreeHostImpl::WindowTreeHostImpl(
 WindowTreeHostImpl::~WindowTreeHostImpl() {
   DestroyCompositor();
   DestroyDispatcher();
+  delete context_factory_;
+  context_factory_ = NULL;
 }
 
 ////////////////////////////////////////////////////////////////////////////////
@@ -159,7 +161,6 @@ ui::EventProcessor* WindowTreeHostImpl::GetEventProcessor() {
 // WindowTreeHostImpl, NativeViewportClient implementation:
 
 void WindowTreeHostImpl::OnCreated(uint64_t native_viewport_id) {
-  LOG(ERROR) << "OnCreated " << native_viewport_id;
   CommandBufferPtr cb;
   gpu_service_->CreateOnscreenGLES2Context(
       native_viewport_id, Size::From(bounds_.size()), Get(&cb));