Create new feature flags for 7z work

We want two flags for this work:
- One is a simple killswitch on the installer code, since breakage
  there is high-impact.
- The other is for an A/B experiment on the Safe Browsing evaluation
  of downloaded 7z archives.

Bug: 1355567
Change-Id: I23fbab7ec73469529e7f2539c085e24c6d7c14be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3888498


Commit-Queue: Daniel Rubery <drubery@chromium.org>
Reviewed-by: thefrog <thefrog@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1045338}