[ios] Allow WebFrame messaging to return results of value 0.

Currently, message.js only sets the response 'result' value
if the return value of a JavaScript function call isn't false,
0, null or undefined. Set the result only if it is not undefined.

Bug: 896867
Change-Id: I9f2211f71ffe9c70b9b4ce85eb949667baa350a6
Reviewed-on: https://chromium-review.googlesource.com/c/1399228
Commit-Queue: Chris Lu <thegreenfrog@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#620954}
1 file changed