[PRESUBMIT] Warning instead of error for deprecated RunLoop methods

pkasting convinced me a while back that forcing an error on these wasn't
great as it blocks refactorings, I've now hit this as well a few times
recently (uploading refactorings with --bypass-hooks and
NOPRESUBMIT=True isn't great)

Warning still encourages developers to do the right thing. While the dev's
decision to continue is unfortunately hidden from the reviewer, these
methods already being named "Deprecated" is a strong enough hint to the
reviewer.

R=dpranke@chromium.org

Bug: None
Change-Id: I29a2624e5217e743ab46799b326577d52cb29d8c
Reviewed-on: https://chromium-review.googlesource.com/1028554Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553731}

PRESUBMIT.py

@@ -425,7 +425,7 @@ _BANNED_CPP_FUNCTIONS = (
         'Please migrate away from RunLoop::QuitCurrent*() methods. Use member',
         'methods of a specific RunLoop instance instead.',
       ),
-      True,
+      False,
       (),
     ),
     (