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Field trials show that this feature greatly improves body reading
performance at the cost of slightly slower following matches.  This
makes sense since the body reading is contending with the later match
operations.

Overall the data suggests this is a net win.  While we don't have a
metric that measures the entire lifecycle of a FetchEvent handled via
cache_storage, we can approximate it by adding the values from these
two UMAs together by percentile slice:

ServiceWorker.LoadTiming.Subresource.ResponseReceivedToCompleted2.CacheStorage
ServiceWorkerCache.Cache.Browser.Match.RelatedFetchEvent

Using this metric we see a 20% or greater progression in median time
to match+read a Response across all platforms.  In addition android saw
a 20% progression at P95.

Bug: 1010624
Change-Id: Id0a68727fe8dd49966988805135a1b03d46dea1f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2343592Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Commit-Queue: Ben Kelly <wanderview@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796104}