Use virtual call to base::Timer::Start

Also update base::Bind to base::BindRepeating.

Background: I tried using the template form of Start() on a base::MockTimer and
was confused because it wasn't working.  This was because Timer::Start() was
getting called instead of MockTimer::Start.

BUG=None
R=thestig

Change-Id: I15341804f364c9d5fd1f590c5e6d02db1728614c
Reviewed-on: https://chromium-review.googlesource.com/1038815
Reviewed-by: Taiju Tsuiki <tzik@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555411}

base/timer/timer.h

@@ -132,8 +132,8 @@ class BASE_EXPORT Timer {
              TimeDelta delay,
              Receiver* receiver,
              void (Receiver::*method)()) {
-    Timer::Start(posted_from, delay,
-                 base::Bind(method, base::Unretained(receiver)));
+    Start(posted_from, delay,
+          base::BindRepeating(method, base::Unretained(receiver)));
   }
 
   // Call this method to stop and cancel the timer.  It is a no-op if the timer