Convert //ios/chrome/browser:about_flags to use buildflag_header.

The template buildflag_header allows generating a header file with
defines based on gn variables. This is better than using "defines"
on a target as only the files using the macro will depend on it
instead of all files in the target.

BUG=None

Review-Url: https://codereview.chromium.org/2569663002
Cr-Commit-Position: refs/heads/master@{#437858}
3 files changed