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The previous URL-keyed metrics landed in
http://crrev.com/c/2237912 fired too often. Update them
to keep track of the slowest serialization and only fire
once every several minutes, or when navigating to a new
page.

Some refactoring was needed to make this testable;
hopefully that should benefit any future tests we write
too.

Bug: 1092944
Change-Id: Icf86f0234b79f15ce27d7ccb9d45841676c1a7d3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2277338Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#787936}