Improve presubmit message for QuitWhenIdleClosure().

Also makes this error about deprecated functions into a warning.  The problems
with using these are not so severe they should block uploads.

BUG=none
TEST=none

Change-Id: I94e6cc900e23d8f3371aace9386a80829906083b
Reviewed-on: https://chromium-review.googlesource.com/902448Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534542}

PRESUBMIT.py

@@ -388,10 +388,10 @@ _BANNED_CPP_FUNCTIONS = (
     (
       'MessageLoop::QuitWhenIdleClosure',
       (
-        'MessageLoop::QuitWhenIdleClosure is deprecated. Please migrate to',
-        'Runloop.',
+        'MessageLoop::QuitWhenIdleClosure is deprecated. Please use a',
+        'QuitWhenIdleClosure obtained from a specific RunLoop instance.',
       ),
-      True,
+      False,
       (),
     ),
     (