syncfs-internals: Replace usages of CallJavascriptFunctionUnsafe.

Replacing with either:
 - ResolveJavascriptCallback where a 1:1 request/response interaction
   between JS and C++ occurs.
 - FireWebUIListener, where C++ -> JS messages are sent.

This is in preparation of migrating chrome://syncfs-internals/ to JS
modules.

Bug: 1154192
Change-Id: I7247e13b286a9926180aa217c352dc9d0bc29021
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2565455
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Commit-Queue: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#832921}