Re-land: cc: Implement shared worker contexts.

cc/test/test_context_provider.h

Index: cc/test/test_context_provider.h
diff --git a/cc/test/test_context_provider.h b/cc/test/test_context_provider.h
index 82b3e612d681344d71d0dbf603f51dcfc8f437a7..3ff9092faece6ec04f27cb4d15bf0441707e7e89 100644
--- a/cc/test/test_context_provider.h
+++ b/cc/test/test_context_provider.h
@@ -25,6 +25,9 @@ class TestContextProvider : public ContextProvider {
     CreateCallback;
 
   static scoped_refptr<TestContextProvider> Create();
+  // Creates a worker context provider that can be used on any thread. This is
+  // equivalent to: Create(); BindToCurrentThread(); SetupLock().
+  static scoped_refptr<TestContextProvider> CreateWorker();
   static scoped_refptr<TestContextProvider> Create(
       scoped_ptr<TestWebGraphicsContext3D> context);