Migrate diagnostics tests off js2gest

- Create source set for url constants to be used in browser tests.
- Convert diagnostics browsertest JS to CC
- Update BUILD.gn files
- Add missing imports causing test failures.

Bug: b:312552350
Test: browser_tests --gtest_filter=DiagnostcsApp*BrowserTest*
Change-Id: I8594e043130d66ce1f8a68ae9af69058b0359563
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5462046


Commit-Queue: Ashley Prasad <ashleydp@google.com>
Reviewed-by: Michael Checo <michaelcheco@google.com>
Cr-Commit-Position: refs/heads/main@{#1289450}