Consolidate string <-> String16 conversions in WebUI js code

This pulls out some frequently copied code that converts js strings to
and from mojo String16 in WebUI js code and migrates callers of all
similar code.

Bug: 1473642
Change-Id: I090cc71e12a9471e6e75ad784ad73ca3ed57546d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4784630


Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Lily Chen <chlily@chromium.org>
Reviewed-by: Jimmy Gong <jimmyxgong@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1184811}