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Adding a UKM similar to Extensions.WebRequest.KeepaliveRequestState.
Proposal: https://docs.google.com/document/d/1XYsa2W1zjWIvrpuAq7IA1wEGZIvbJLoiPF6rMob01Tg/edit

Bug: 1098251
Change-Id: I6bccb85935646bb9ed1879ec19fb409323807124
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2262402
Auto-Submit: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#782836}