Make build/include_what_you_use more consistent

This is a cherry-pick of https://github.com/google/styleguide/pull/158/

BUG=627202
TEST=cpplint_unittest.py

Review-Url: https://codereview.chromium.org/2147593002

cpplint.py

@@ -5415,7 +5415,12 @@ def CheckForIncludeWhatYouUse(filename, clean_lines, include_state, error,
       continue
 
     for pattern, template, header in _re_pattern_templates:
-      if pattern.search(line):
+      matched = pattern.search(line)
+      if matched:
+        # Don't warn about IWYU in non-STL namespaces:
+        # (We check only the first match per line; good enough.)
+        prefix = line[:matched.start()]
+        if prefix.endswith('std::') or not prefix.endswith('::'):
           required[header] = (linenum, template)
 
   # The policy is that if you #include something in foo.h you don't need to