Only presubmit warn when spammy logging is being added/modified.

PRESUBMIT.py

 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Top-level presubmit script for Chromium.
 
 See http://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts
 for more details about the presubmit API built into gcl.
 """
 
@@ skipping 991 matching lines @@
                  r"^remoting[\\\/]base[\\\/]logging\.h$",
                  r"^remoting[\\\/]host[\\\/].*",
                  r"^sandbox[\\\/]linux[\\\/].*",
                  r"^tools[\\\/]",
                  r"^ui[\\\/]aura[\\\/]bench[\\\/]bench_main\.cc$",
                  r"^webkit[\\\/]browser[\\\/]fileapi[\\\/]" +
                      r"dump_file_system.cc$",))
   source_file_filter = lambda x: input_api.FilterSourceFile(
       x, white_list=(file_inclusion_pattern,), black_list=black_list)
 
+  log_macro = input_api.re.compile(r"\bD?LOG\s*\(\s*INFO\s*\)")
+  log_if_macro = input_api.re.compile(r"\bD?LOG_IF\s*\(\s*INFO\s*,")
+  printf_macro = input_api.re.compile(r"\bprintf\(")
+  fprintf_macro = input_api.re.compile(r"\bfprintf\((stdout|stderr)")
+
   log_info = []
   printf = []
 
   for f in input_api.AffectedSourceFiles(source_file_filter):
-    contents = input_api.ReadFile(f, 'rb')
-    if input_api.re.search(r"\bD?LOG\s*\(\s*INFO\s*\)", contents):
-      log_info.append(f.LocalPath())
-    elif input_api.re.search(r"\bD?LOG_IF\s*\(\s*INFO\s*,", contents):
-      log_info.append(f.LocalPath())
-
-    if input_api.re.search(r"\bprintf\(", contents):
-      printf.append(f.LocalPath())
-    elif input_api.re.search(r"\bfprintf\((stdout|stderr)", contents):
-      printf.append(f.LocalPath())
+    for linenum, line in f.ChangedContents():
+      if log_macro.search(line) or log_if_macro.search(line):
+        log_info.append(f.LocalPath())
+      if printf_macro.search(line) or fprintf_macro.search(line):
+        printf.append(f.LocalPath())
 
   if log_info:
     return [output_api.PresubmitError(
       'These files spam the console log with LOG(INFO):',
       items=log_info)]
   if printf:
     return [output_api.PresubmitError(
       'These files spam the console log with printf/fprintf:',
       items=printf)]
   return []
@@ skipping 561 matching lines @@
   builders = [
       'Mojo Linux Try',
       'Mojo Linux (dbg) Try',
       'Mojo Android Builder Try',
       'Mojo Android Builder (dbg) Try',
       'Mojo ChromeOS Builder Try',
       'Mojo ChromeOS Builder (dbg) Try',
   ]
 
   return GetDefaultTryConfigs(builders)