[Mojo] Fix conversion js strings to/from mojo String16 for Unicode.

- stringToMojoString16

  - this method was pulling the wrong character codes for multi code
point characters, resulting in improper output
  - instead, iterate over each code point  to get the correct output.


- mojoString16ToString
  - Instead of using to fromcodepoint, use fromCharCode since thats what
mojo accepts,

Tests are provided which convert emojis including graphemes back and
forth.

Change-Id: Iaa7bff70553a30f59772949aac4fb8ea31569ec1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5027656
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Eshwar Stalin <estalin@chromium.org>
Auto-Submit: David Pennington <dpenning@chromium.org>
Commit-Queue: David Pennington <dpenning@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1225732}

chrome/test/data/webui/js/mojo_type_util_test.ts

@@ -18,6 +18,19 @@ suite('MojoTypeUtilTest', () => {
     assertEquals(mojoString16ToString({data: [0x4f60, 0x597d]}), '你好');
   });
 
+  test('emojis', () => {
+    assertEquals('❤️', mojoString16ToString(stringToMojoString16('❤️')));
+    assertEquals(
+        '👨‍👨‍👦',
+        mojoString16ToString(stringToMojoString16('👨‍👨‍👦')));
+    assertEquals('🇯🇵', mojoString16ToString(stringToMojoString16('🇯🇵')));
+    assertEquals('🇺🇳', mojoString16ToString(stringToMojoString16('🇺🇳')));
+    assertEquals(
+        '👨‍👨‍👦🇯🇵👨‍👨‍👦a你❤️👨‍👨‍👦',
+        mojoString16ToString(stringToMojoString16(
+            '👨‍👨‍👦🇯🇵👨‍👨‍👦a你❤️👨‍👨‍👦')));
+  });
+
   test('Can convert strings to mojo Urls', () => {
     assertDeepEquals(stringToMojoUrl(''), {url: ''});
     assertDeepEquals(

ui/webui/resources/js/mojo_type_util.ts

@@ -5,12 +5,18 @@
 import {String16} from 'chrome://resources/mojo/mojo/public/mojom/base/string16.mojom-webui.js';
 import {Url} from 'chrome://resources/mojo/url/mojom/url.mojom-webui.js';
 
-export function stringToMojoString16(s: string): String16 {
-  return {data: Array.from(s, c => c.charCodeAt(0))};
+// Convert a javascript string into a Mojo String16.
+export function stringToMojoString16(str: string): String16 {
+  const arr: number[] = [];
+  for (let i = 0; i < str.length; i++) {
+    arr.push(str.charCodeAt(i));
+  }
+  return {data: arr};
 }
 
+// Convert a Mojo String16 into a javascript string.
 export function mojoString16ToString(str16: String16): string {
-  return str16.data.map((ch: number) => String.fromCodePoint(ch)).join('');
+  return String.fromCharCode(...str16.data);
 }
 
 // Note: This does not do any validation of the URL string.