Update Google API keys header comment.

google_apis/google_api_keys.h

 // 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.
 
 #ifndef GOOGLE_APIS_GOOGLE_API_KEYS_H_
 #define GOOGLE_APIS_GOOGLE_API_KEYS_H_
 
 // If you add more includes to this file, you also need to add them to
 // google_api_keys_unittest.cc.
 #include <string>
 
 // These functions enable you to retrieve keys to use for Google APIs
 // such as Translate and Safe Browsing.
 //
 // You can retrieve either an "API key" (sometimes called a developer
 // key) which identifies you (or the company you work for) as a
 // developer, or you can retrieve the "client ID" and "client secret"
 // used by you (or the company you work for) to generate OAuth2
 // requests.
 //
 // Each developer (or group of developers working together for a
 // single company) must request a Google API key and the client ID and
 // client secret for OAuth2.  See
 // https://developers.google.com/console/help/ and
 // https://developers.google.com/console/.
 //
 // The keys must either be provided using preprocessor variables (set
-// via e.g. ~/.gyp/include.gypi). Alternatively, they can be
+// via e.g. ~/.gyp/include.gypi). Alternatively, in Chromium builds, they can be
 // overridden at runtime using environment variables of the same name.
+// Environment variable overrides will be ignored for official Google Chrome
+// builds.
 //
 // The names of the preprocessor variables (or environment variables
-// to override them at runtime) are as follows:
+// to override them at runtime in Chromium builds) are as follows:
 // - GOOGLE_API_KEY: The API key, a.k.a. developer key.
 // - GOOGLE_DEFAULT_CLIENT_ID: If set, this is used as the default for
 //   all client IDs not otherwise set.  This is intended only for
 //   development.
 // - GOOGLE_DEFAULT_CLIENT_SECRET: If set, this is used as the default
 //   for all client secrets.  This is intended only for development.
 // - GOOGLE_CLIENT_ID_[client name]
 //   (e.g. GOOGLE_CLIENT_ID_CLOUD_PRINT, i.e. one for each item in the
 //   OAuth2Client enumeration below)
 // - GOOGLE_CLIENT_SECRET_[client name]
@@ skipping 73 matching lines @@
 // Returns the auth token for the data reduction proxy.
 std::string GetSpdyProxyAuthValue();
 
 // Returns if the API key using in the current build is the one for official
 // Google Chrome.
 bool IsGoogleChromeAPIKeyUsed();
 
 }  // namespace google_apis
 
 #endif  // GOOGLE_APIS_GOOGLE_API_KEYS_H_