Add Custom Feeback Flag to about://flags

To setup for testing both with the testing, accessiblity and security
teams the feedback UI needs to be publically acessible. This CL switches
the existing flag from a finch flag to a ui flag.

BUG=613357

Review-Url: https://codereview.chromium.org/2722603005
Cr-Commit-Position: refs/heads/master@{#454157}
3 files changed