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This patch fixes the crash on debugging build when visiting chrome://ukm
due to CheckFeatureIdentity(UkmSamplingRate) failure.
COMPONENT_EXPORT makes the definition of the base::Feature
UkmSamplingRate from ukm_recorder_impl.cc exported and shared by the
libraries that reference it, ensuring there's only one copy of the
base::Feature in memory.

Bug-id: b/160004246
Change-Id: Icda0e0a403538a263f8132afc19dd0cb455395db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2274240
Commit-Queue: Yue Ru Sun <yrsun@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#784145}