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Feature params cannot change at runtime. Retrieve them only once,
instead of repeatedly.

We're seeing lots of crashes with SIGABRT on shutdown retrieving
UKM feature params by these code paths. SIGABRT may suggest that
it's just shutdown taking too long and it just happens to land
here (because retrieving params takes a bit of time). Eliminate
this possibility by caching these values.

This should slightly speed up UKM recording, including on shut
down. If shut down still takes too long and is the cause of the
SIGABRTs we're seeing, the stack traces should hopefully change
and the field trial param code won't be blamed anymore (perhaps
replaced by some other slow code, which can be fixed separately).

Bug: 1131877
Change-Id: I365e7a6fac5d3ef941cd0a26e9b280d03df6b28b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2615820
Commit-Queue: Robert Kaplow <rkaplow@chromium.org>
Auto-Submit: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841485}