Add UTF8String to the banned method list.

NSString's UTF8String can return null for some NSStrings, even if
|canBeConvertedToEncoding:NSUTF8StringEncoding| returns YES.
This has led to at least 2 crashes that were not encountered during
the development period.

BUG=659649, 656108, 653379

Review-Url: https://codereview.chromium.org/2450033004
Cr-Commit-Position: refs/heads/master@{#427991}

PRESUBMIT.py

@@ -147,6 +147,15 @@
       ),
       True,
     ),
+    (
+      r"/\s+UTF8String\s*]",
+      (
+       'The use of -[NSString UTF8String] is dangerous as it can return null',
+       'even if |canBeConvertedToEncoding:NSUTF8StringEncoding| returns YES.',
+       'Please use |SysNSStringToUTF8| instead.',
+      ),
+      True,
+    ),
 )