GitLab

- Simplifies implementing fake observers
- Currently only supports a single observable value

Bug: 1125150
Test: browser_tests --gtest_filter=DiagnosticsApp*
Change-Id: Idcddbbe30e220ecab8e7218450630062fb69c8e1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2420769Reviewed-by: Bailey Berro <baileyberro@chromium.org>
Commit-Queue: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809588}