Do not serialize remote objects unless needed

This patch stops serializing console API arguments unless there are
listeners of the 'console' event in puppeteer.

This saves quite a lot CPU cycles.

Fixes #117.

lib/Page.js

@@ -232,6 +232,10 @@ class Page extends EventEmitter {
       }
       return;
     }
+    if (!this.listenerCount(Page.Events.Console)) {
+      event.args.map(arg => helper.releaseObject(this._client, arg));
+      return;
+    }
     let values = await Promise.all(event.args.map(arg => helper.serializeRemoteObject(this._client, arg)));
     this.emit(Page.Events.Console, ...values);
   }

lib/helper.js

@@ -45,7 +45,7 @@ class Helper {
   /**
    * @param {!Connection} client
    * @param {!Object} remoteObject
-   * @return {!Object}
+   * @return {!Promise<!Object>}
    */
   static async serializeRemoteObject(client, remoteObject) {
     if (remoteObject.unserializableValue) {
@@ -75,9 +75,23 @@ class Helper {
       // Return description for unserializable object, e.g. 'window'.
       return remoteObject.description;
     } finally {
-      client.send('Runtime.releaseObject', {objectId: remoteObject.objectId}).catch(e => {
-        // While we were serializing object, the page might've navigated.
-      });
+      Helper.releaseObject(client, remoteObject);
+    }
+  }
+
+  /**
+   * @param {!Connection} client
+   * @param {!Object} remoteObject
+   * @return {!Promise}
+   */
+  static async releaseObject(client, remoteObject) {
+    if (!remoteObject.objectId)
+      return;
+    try {
+      await client.send('Runtime.releaseObject', {objectId: remoteObject.objectId});
+    } catch (e) {
+      // Exceptions might happen in case of a page been navigated or closed.
+      // Swallow these since they are harmless and we don't leak anything in this case.
     }
   }