Change <emoji-picker> to have emoji-data-url attribute.

Defaults to the current emoji ordering from emoji-metadata,
but this will allow smaller emoji data files to be used for tests.

Change-Id: Ibc3ffdeb0c05f7ad9c3880c899c92ef6dc6cbcbd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2639713Reviewed-by: John Palmer <jopalmer@chromium.org>
Reviewed-by: Keith Lee <keithlee@chromium.org>
Commit-Queue: Kenton Lam <kentonlam@google.com>
Cr-Commit-Position: refs/heads/master@{#845403}

chrome/browser/resources/chromeos/emoji_picker/emoji_picker.js

@@ -38,6 +38,7 @@ class EmojiPicker extends PolymerElement {
 
   static get properties() {
     return {
+      emojiDataUrl: {type: String, value: EMOJI_ORDERING_JSON},
       groups: {type: Array},
       /** @type {?EmojiData} */
       emojiData: {
@@ -78,7 +79,7 @@ class EmojiPicker extends PolymerElement {
 
     const xhr = new XMLHttpRequest();
     xhr.onloadend = () => this.onEmojiDataLoaded(xhr.responseText);
-    xhr.open('GET', EMOJI_ORDERING_JSON);
+    xhr.open('GET', this.emojiDataUrl);
     xhr.send();
 
     this.addEventListener(