Move WeakPtrFactory member last in class

It was failing like this:

In file included from ../../media/mojo/services/mojo_cdm_service.cc:5:
../../media/mojo/services/mojo_cdm_service.h:71:40: error: [chromium-style] WeakPtrFactory members which refer to their outer class must be the last member in the outer class definition.
  base::WeakPtrFactory<MojoCdmService> weak_factory_;
                                       ^
BUG=444089

Review URL: https://codereview.chromium.org/872773008

Cr-Commit-Position: refs/heads/master@{#313796}

media/mojo/services/mojo_cdm_service.h

@@ -68,8 +68,8 @@ class MojoCdmService
 
   scoped_ptr<MediaKeys> cdm_;
 
-  base::WeakPtrFactory<MojoCdmService> weak_factory_;
   base::WeakPtr<MojoCdmService> weak_this_;
+  base::WeakPtrFactory<MojoCdmService> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(MojoCdmService);
 };