GitLab

This reverts commit bd1cc624.

Original CL:
Refactor ExtensionManifestTest to allow usage in src/extensions
https://codereview.chromium.org/572813002

Reason:
Broke the official Win builders due to differences in how they report their
version channel.
http://master.chrome.corp.google.com:8011/builders/win%20trunk/builds/26150

BUG=414917
TEST=unit_tests
TBR=rockot@chromium.org

Review URL: https://codereview.chromium.org/578653005

Cr-Commit-Position: refs/heads/master@{#295172}