Allow chrome.debugger API to attach to extension background pages.

chrome/browser/extensions/api/debugger/debugger_api_constants.h

Index: chrome/browser/extensions/api/debugger/debugger_api_constants.h
diff --git a/chrome/browser/extensions/api/debugger/debugger_api_constants.h b/chrome/browser/extensions/api/debugger/debugger_api_constants.h
index 67e605e9ef494c1393d9f1b9f881fef2a3206565..8178fdd521d8e3baec680d1eb85e72ef02b93414 100644
--- a/chrome/browser/extensions/api/debugger/debugger_api_constants.h
+++ b/chrome/browser/extensions/api/debugger/debugger_api_constants.h
@@ -16,10 +16,15 @@ extern const char kOnDetach[];
 // Errors.
 extern const char kAlreadyAttachedError[];
 extern const char kAttachToWebUIError[];
-extern const char kNoTabError[];
+extern const char kNoTargetError[];
+extern const char kInvalidTargetError[];
 extern const char kNotAttachedError[];
 extern const char kProtocolVersionNotSupportedError[];
 
+// Target types.
+extern const char kTabTargetType[];
+extern const char kExtensionTargetType[];
+
 }  // namespace debugger_api_constants
 
 #endif  // CHROME_BROWSER_EXTENSIONS_API_DEBUGGER_DEBUGGER_API_CONSTANTS_H_