Add PRESUBMIT to disallow new usage of FOR_EACH_OBSERVER.

PRESUBMIT.py

Index: PRESUBMIT.py
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 3083e5455a90c559456b1d4a5eb31dd3eeb6381e..ac5d014e5b2a061a05b624c21f06662dc239dd35 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -307,6 +307,15 @@ _BANNED_CPP_FUNCTIONS = (
       True,
       (),
     ),
+    (
+      r'FOR_EACH_OBSERVER(',
+      (
+        'Use range-based for loops to iterate through base::ObserverList, e.g.',
+        '        for (auto& observer : observers) { observer->Observe(); }',

[loyso (OOO), 2016/10/12 06:27:24]

observer.Observe();

[dcheng, 2016/10/12 06:36:12]

Oops done.

+      ),
+      True,
+      (),
+    ),
 )