Remove promise jsdoc (#641)

This patch removes unnecessary "promise" return types in jsdoc for async functions.

lib/FrameManager.js

@@ -284,7 +284,7 @@ class Frame {
 
   /**
    * @param {string} filePath
-   * @return {!Promise}
+   * @return {!Promise<*>}
    */
   async injectFile(filePath) {
     let contents = await new Promise((resolve, reject) => {
@@ -299,7 +299,6 @@ class Frame {
 
   /**
    * @param {string} url
-   * @return {!Promise}
    */
   async addScriptTag(url) {
     return this.evaluate(addScriptTag, url);