Defer compositor context creation to the thread.

cc/output/context_provider.h

Index: cc/output/context_provider.h
diff --git a/cc/output/context_provider.h b/cc/output/context_provider.h
index 2245b643e134b464d0db441cd4bd5dfae0ca2d2e..dadda38e4f5247a8c1c52a8adc5b67c0553c6716 100644
--- a/cc/output/context_provider.h
+++ b/cc/output/context_provider.h
@@ -57,13 +57,21 @@ class ContextProvider : public base::RefCountedThreadSafe<ContextProvider> {
     base::AutoLock context_lock_;
   };
 
-  // Bind the 3d context to the current thread. This should be called before
-  // accessing the contexts. Calling it more than once should have no effect.
-  // Once this function has been called, the class should only be accessed
-  // from the same thread unless the function has some explicitly specified
-  // rules for access on a different thread. See SetupLockOnMainThread(), which
-  // can be used to provide access from multiple threads.
-  virtual bool BindToCurrentThread() = 0;
+  // This Factory API is how to create a context provider in a deferred way (ie.
+  // on another thread than the main thread).
+  class DeferredCreate {

[piman, 2016/05/17 03:41:40]

naming-wise, how about Factory? It's shorter, and I think it carries the right
idea?

[danakj, 2016/05/17 19:53:32]

Ya it does. I was like ContextProvider::Factory is very close to
ContextProvider::Provider, but whatever, embrace it I guess.

+   public:
+    // Creates and returns a new context provider. The context provider may only
+    // be used on the thread it is created on unless it supports locking (via
+    // the ScopedContextLock class).
+    virtual scoped_refptr<ContextProvider> CreateContext() = 0;
+
+   protected:
+    DeferredCreate() {}
+
+   private:
+    DISALLOW_COPY_AND_ASSIGN(DeferredCreate);
+  };
 
   virtual gpu::gles2::GLES2Interface* ContextGL() = 0;
   virtual gpu::ContextSupport* ContextSupport() = 0;