[api] add Puppeteer.executablePath() method (#771)

This patch adds Puppeteer.executablePath() method to query the path
of bundled chromium.

Fixes #745

lib/Launcher.js

@@ -111,6 +111,14 @@ class Launcher {
     }
   }
 
+  /**
+   * @return {string}
+   */
+  static executablePath() {
+    const revisionInfo = Downloader.revisionInfo(Downloader.currentPlatform(), ChromiumRevision);
+    return revisionInfo.executablePath;
+  }
+
   /**
    * @param {string} options
    * @return {!Promise<!Browser>}