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 4e3850febc071865acdb902fafcf1f71c9e845d9..9b6bce1bba7bb25bf12ff0ec801439b6991a5155 100644
--- a/cc/trees/layer_tree_host_unittest.cc
+++ b/cc/trees/layer_tree_host_unittest.cc
@@ -464,7 +464,7 @@ 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));
@@ -6189,5 +6189,21 @@ 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