Re-land: cc: Implement shared worker contexts.

cc/trees/layer_tree_host_unittest.cc

Index: cc/trees/layer_tree_host_unittest.cc
diff --git a/cc/trees/layer_tree_host_unittest.cc b/cc/trees/layer_tree_host_unittest.cc
index 1481b99550feb70749e4acee17f94a34ffad82d9..0afcd9fa7400356395a3d02104e3b234390fdedb 100644
--- a/cc/trees/layer_tree_host_unittest.cc
+++ b/cc/trees/layer_tree_host_unittest.cc
@@ -465,7 +465,7 @@ class LayerTreeHostFreeWorkerContextResourcesTest : public LayerTreeHostTest {
     explicit MockSetWorkerContextShouldAggressivelyFreeResourcesOutputSurface(
         bool delegated_rendering)
         : FakeOutputSurface(TestContextProvider::Create(),
-                            TestContextProvider::Create(),
+                            TestContextProvider::CreateWorker(),
                             delegated_rendering) {}
     MOCK_METHOD1(SetWorkerContextShouldAggressivelyFreeResources,
                  void(bool is_visible));
@@ -6258,5 +6258,21 @@ class LayerTreeHostScrollingAndScalingUpdatesLayers : public LayerTreeHostTest {
 
 MULTI_THREAD_TEST_F(LayerTreeHostScrollingAndScalingUpdatesLayers);
 
+class LayerTreeHostTestDestroyWhileInitializingOutputSurface
+    : public LayerTreeHostTest {
+ protected:
+  void BeginTest() override {
+    // By ending the test immediately we start initialization of an output
+    // surface but destroy the LTH before it completes. This test verifies
+    // that this works correctly and the output surface is destroyed on
+    // the correct thread.
+    EndTest();
+  }
+
+  void AfterTest() override {}
+};
+
+MULTI_THREAD_TEST_F(LayerTreeHostTestDestroyWhileInitializingOutputSurface);
+
 }  // namespace
 }  // namespace cc