Fix -Wdefaulted-function-deleted warning in MessageLoopCurrent

The new Clang warning points out that the class can't be copy
assigned because the current_ member is const. Copy or move
constructing it is fine though, and that's all that's needed.

Bug: 890307
Change-Id: I3f4d5e69485b84166ba4dd2356cc7973a5e58da6
Reviewed-on: https://chromium-review.googlesource.com/1255613Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595867}

base/message_loop/message_loop_current.h

@@ -38,9 +38,9 @@ class MessageLoop;
 class BASE_EXPORT MessageLoopCurrent {
  public:
   // MessageLoopCurrent is effectively just a disguised pointer and is fine to
-  // copy around.
+  // copy/move around.
   MessageLoopCurrent(const MessageLoopCurrent& other) = default;
-  MessageLoopCurrent& operator=(const MessageLoopCurrent& other) = default;
+  MessageLoopCurrent(MessageLoopCurrent&& other) = default;
 
   // Returns a proxy object to interact with the MessageLoop running the
   // current thread. It must only be used on the thread it was obtained.