Initial CL for Data Saver (Flywheel) prompt when cellular network detected

chrome/common/extensions/extension_constants.h

Index: chrome/common/extensions/extension_constants.h
diff --git a/chrome/common/extensions/extension_constants.h b/chrome/common/extensions/extension_constants.h
index e97794ab3d833c0450834958bee9122488e086ca..b368a7d49a970f16a84ccda65f8e5acbf3f6eb51 100644
--- a/chrome/common/extensions/extension_constants.h
+++ b/chrome/common/extensions/extension_constants.h
@@ -37,6 +37,9 @@ extern const char kChromeAppId[];
 // The extension id of the Cloud Print component application.
 extern const char kCloudPrintAppId[];
 
+// The extension id of the Data Saver extension.
+extern const char kDataSaverExtensionId[];
+
 // The extension id of the Easy Unlock component application.
 extern const char kEasyUnlockAppId[];