Mark hasListener as returning a boolean

chrome/chrome.d.ts

@@ -869,7 +869,7 @@ declare module chrome.events {
         addListener(callback: Function): void;
         getRules(callback: (rules: Rule[]) => void): void;
         getRules(ruleIdentifiers: string[], callback: (rules: Rule[]) => void): void;
-        hasListener(callback: Function): void;
+        hasListener(callback: Function): boolean;
         removeRules(ruleIdentifiers?: string[], callback?: Function): void;
         addRules(rules: Rule[], callback?: (rules: Rule[]) => void): void;
         removeListener(callback: Function): void;