Fix the response check.

Response from Mojo service is a dict with success as its key, which
is defined in input_engine.mojom under chromeos/services/ime

Bug: 837156
Change-Id: Ie33c5aaed27c3de8b995b8455b65074b6bbe2cba
Reviewed-on: https://chromium-review.googlesource.com/c/1460656Reviewed-by: Darren Shen <shend@chromium.org>
Commit-Queue: Leo Zhang <googleo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630695}

chrome/renderer/resources/extensions/chromeos_ime_service_bindings.js

@@ -255,7 +255,8 @@ class ImeService {
           .connectToImeEngine(
               imeSpec, mojo.makeRequest(this.activeEngine_),
               this.clientChannel_.getChannelPtr(), extra)
-          .then((bound) => {
+          .then((result) => {
+            const bound = result && result['success'];
             if (bound && onConnectionError) {
               this.activeEngine_.ptr.setConnectionErrorHandler(
                   onConnectionError);