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This CL is a pure refactoring. After this change,
BackForwardCacheMetrics can take the full information about storing a
document into the cache.

Bug: 1004676
Change-Id: Ie56dfde2f2d5e0cb6d8507607dfd38f61837a358
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1899630
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712914}