BrowserThemePack: Fix GCC build after #581022

Due to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84849, having
base::NoDestructor<T<U>> and passing an initializer list of Us does not
work.

Just explicitly create a base::flat_set in the constructor. See also:
https://chromium-review.googlesource.com/1165360

Bug: 819294
Change-Id: I24593dc199df47c2f8fd316436517704d159e4fa
Reviewed-on: https://chromium-review.googlesource.com/1170905
Commit-Queue: Raphael Kubo da Costa (CET) <raphael.kubo.da.costa@intel.com>
Reviewed-by: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582625}
1 file changed