clang/win: Make diagnostics colored if ansicon is around at runhooks time.

BUG=82385
TEST=Run gyp or gn with ansicon active, build. Diagnostics should be colored
in cmd.exe.

Review URL: https://codereview.chromium.org/1194823004

Cr-Original-Commit-Position: refs/heads/master@{#335394}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 59fc7718fefd2c93e1acd94c4d5a405949a0cf10

common.gypi

@@ -5760,6 +5760,17 @@
                 ],
               },
             }],
+            ['clang==1 and "<!(python <(DEPTH)/build/win/use_ansi_codes.py)"=="True"', {
+              'VCCLCompilerTool': {
+                'AdditionalOptions': [
+                  # cmd.exe doesn't understand ANSI escape codes by default,
+                  # so only enable them if something emulating them is around.
+                  '-fansi-escape-codes',
+                  # Also see http://crbug.com/110262
+                  '-fcolor-diagnostics',
+                ],
+              },
+            }],
           ],
         },
       },

config/compiler/BUILD.gn

@@ -130,6 +130,14 @@ config("compiler") {
       } else {
         cflags += [ "-m64" ]
       }
+      if (exec_script("//build/win/use_ansi_codes.py", [], "trim string") ==
+          "True") {
+        cflags += [
+          # cmd.exe doesn't understand ANSI escape codes by default,
+          # so only enable them if something emulating them is around.
+          "-fansi-escape-codes",
+        ]
+      }
     }
   } else {
     # Common GCC compiler flags setup.

win/use_ansi_codes.py

@@ -0,0 +1,10 @@
+#!/usr/bin/env python
+# Copyright (c) 2015 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.
+"""Prints if the the terminal is likely to understand ANSI codes."""
+
+import os
+
+# Add more terminals here as needed.
+print 'ANSICON' in os.environ