Make RunLoop::Delegate methods public.

This mimics the pre-existing paradigm for MessagePump::Delegate :
public methods on Delegate which are then privately inherited.

There were cr-dev discussions on these topics (composition vs private
inheritance) but I don't see a reason to deviate from the
pre-established MessagePump paradigm here.
https://groups.google.com/a/chromium.org/d/topic/chromium-dev/XdsXTHy9lis/discussion
https://groups.google.com/a/chromium.org/d/topic/chromium-dev/CwzjcWd9cYM/discussion

I did consider composition here (e.g. with a private RunDelegate
class in MessageLoop, but the many outer_-> in
Run/Quit/EnsureWorkScheduled made it ugly.

R=danakj@chromium.org

Bug: 708584
Change-Id: I42830b3a28d29b1a518db4cfc767a655de491a90
Reviewed-on: https://chromium-review.googlesource.com/798290Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521227}

base/run_loop.h

@@ -157,7 +157,7 @@ class BASE_EXPORT RunLoop {
   // via RunLoop::RegisterDelegateForCurrentThread() before RunLoop instances
   // and RunLoop static methods can be used on it.
   class BASE_EXPORT Delegate {
-   protected:
+   public:
     Delegate();
     ~Delegate();
 
@@ -183,11 +183,6 @@ class BASE_EXPORT RunLoop {
       Delegate* outer_;
     };
 
-   private:
-    // While the state is owned by the Delegate subclass, only RunLoop can use
-    // it.
-    friend class RunLoop;
-
     // Used by RunLoop to inform its Delegate to Run/Quit. Implementations are
     // expected to keep on running synchronously from the Run() call until the
     // eventual matching Quit() call. Upon receiving a Quit() call it should
@@ -212,6 +207,11 @@ class BASE_EXPORT RunLoop {
     // system messages.
     virtual void EnsureWorkScheduled() = 0;
 
+   private:
+    // While the state is owned by the Delegate subclass, only RunLoop can use
+    // it.
+    friend class RunLoop;
+
     // A vector-based stack is more memory efficient than the default
     // deque-based stack as the active RunLoop stack isn't expected to ever
     // have more than a few entries.