cc: Implement shared worker contexts. (v1)

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 c024e02328e9a3930b7593e54b986bdff3566ae5..b3e721498334dee5b1e85d9db09d0cf81f8f8284 100644
--- a/cc/output/context_provider.h
+++ b/cc/output/context_provider.h
@@ -32,16 +32,20 @@ class ContextProvider : public base::RefCountedThreadSafe<ContextProvider> {
     explicit ScopedContextLock(ContextProvider* context_provider)
         : context_provider_(context_provider),
           context_lock_(*context_provider_->GetLock()) {
-      // Allow current thread to bind to |context_provider|.
+      // Allow current thread to use |context_provider_|.
       context_provider_->DetachFromThread();
     }
     ~ScopedContextLock() {
-      // Allow a different thread to bind to |context_provider|.
+      // Allow usage by thread for which |context_provider_| is bound to.
       context_provider_->DetachFromThread();
     }
 
+    // Returns a raw pointer to the GLES2Interface or null if context has been
+    // destroyed.
     gpu::gles2::GLES2Interface* ContextGL() {
-      return context_provider_->ContextGL();
+      return context_provider_->HasBeenDestroyed()
+                 ? nullptr
+                 : context_provider_->ContextGL();
     }
 
    private:
@@ -107,6 +111,9 @@ class ContextProvider : public base::RefCountedThreadSafe<ContextProvider> {
   virtual void SetMemoryPolicyChangedCallback(
       const MemoryPolicyChangedCallback& memory_policy_changed_callback) = 0;
 
+  // Return true if the context inside the provider is no longer valid.
+  virtual bool HasBeenDestroyed() = 0;
+
  protected:
   friend class base::RefCountedThreadSafe<ContextProvider>;
   virtual ~ContextProvider() {}