UkmFeatures: Fix GCC build after #598120

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/c/chromium/src/+/1170905

Bug: 819294
Change-Id: I887b461bc1263af02ee9dc448aea1d9ff6d1e5c7
Reviewed-on: https://chromium-review.googlesource.com/c/1273746
Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Raphael Kubo da Costa (CET) <raphael.kubo.da.costa@intel.com>
Cr-Commit-Position: refs/heads/master@{#598517}
1 file changed