Added frame.waitFor(selector) (#59)

This patch adds `frame.waitFor(selector)` method.

Fixes #42.

lib/FrameManager.js

@@ -197,6 +197,39 @@ class FrameManager extends EventEmitter {
     });
     return response.result.value;
   }
+
+  /**
+   * @param {string} selector
+   * @param {!Frame} frame
+   * @return {!Promise<undefined>}
+   */
+  async _waitForInFrame(selector, frame) {
+    let code = selector => new Promise((fulfill, reject) => {
+      if (document.querySelector(selector)) {
+        fulfill();
+        return;
+      }
+      new MutationObserver((mutations, observer) => {
+        for (let mutation of mutations) {
+          for (let node of mutation.addedNodes) {
+            if (node.matches(selector)) {
+              observer.disconnect();
+              fulfill();
+              return;
+            }
+          }
+        }
+      }).observe(document.documentElement, {
+        childList: true,
+        subtree: true
+      });
+    });
+    await this._client.send('Runtime.evaluate', {
+      expression: helper.evaluationString(code, selector),
+      awaitPromise: true,
+      returnByValue: true,
+    });
+  }
 }
 
 /** @enum {string} */
@@ -287,6 +320,14 @@ class Frame {
     return this._detached;
   }
 
+  /**
+   * @param {string} selector
+   * @return {!Promise<undefined>}
+   */
+  async waitFor(selector) {
+    await this._frameManager._waitForInFrame(selector, this);
+  }
+
   /**
    * @param {?Object} framePayload
    */