Add gpu::MailboxHolder to hold state for a gpu::Mailbox

content/common/gpu/client/gl_helper.h

Index: content/common/gpu/client/gl_helper.h
diff --git a/content/common/gpu/client/gl_helper.h b/content/common/gpu/client/gl_helper.h
index bac0560651b048f6a8b819a781ecf02f5c9eda04..21bbcaf1599039bd6902b3c40e5a587fae409b2d 100644
--- a/content/common/gpu/client/gl_helper.h
+++ b/content/common/gpu/client/gl_helper.h
@@ -296,12 +296,11 @@ class CONTENT_EXPORT GLHelper {
   // Wait for the sync point before executing further GL commands.
   void WaitSyncPoint(uint32 sync_point);
 
-  // Creates a mailbox that is attached to the given texture id, and a sync
-  // point to wait on before using the mailbox. Returns an empty mailbox on
-  // failure.
+  // Creates a mailbox holder that is attached to the given texture id, with a
+  // sync point to wait on before using the mailbox. Returns a holder with an
+  // empty mailbox on failure.
   // Note the texture is assumed to be GL_TEXTURE_2D.
-  gpu::Mailbox ProduceMailboxFromTexture(blink::WebGLId texture_id,
-                                         uint32* sync_point);
+  gpu::MailboxHolder ProduceMailboxHolderFromTexture(blink::WebGLId texture_id);
 
   // Creates a texture and consumes a mailbox into it. Returns 0 on failure.
   // Note the mailbox is assumed to be GL_TEXTURE_2D.