Properly decode JS message before executing.

The command parameter can contain not ascii characters
which will not be decoded correctly by fromCharCode.
Using TextDecoder will be more robust to decode the
string.

Bug: 950538
Change-Id: Ia7495c328582585ab8d47038b3c0bd192a40fe37
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1556848
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#648736}

ios/web/web_state/js/resources/message.js

@@ -273,8 +273,7 @@ var executeMessage_ = function(payload, iv) {
   getFrameSymmetricKey_(function(frameKey) {
     window.crypto.subtle.decrypt(algorithm, frameKey, encryptedFunctionArray)
         .then(function(decrypted) {
-      var callJSON =
-          String.fromCharCode.apply(null, new Uint8Array(decrypted));
+      var callJSON = new TextDecoder().decode(new Uint8Array(decrypted));
       var callDict = JSON.parse(callJSON);
 
       // Verify that message id is valid.