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This patch implements the following functionality:
- Adding an entry to the cache
- Restoring an entry from the cache
- Evicting an entry by SessionID
- Getting the time to live in the cache
- Getting a pointer to an entry from a given SessionID
- Evicting an entry after timeout

Design doc:
https://docs.google.com/document/d/1SF230MYWgroe4WikDMn82ETd3-HFRzylKtIk0HZRaOU/edit?usp=sharing

BUG=1100946

Change-Id: Ic1f137c255a9695df080868d217cd3fec7d18690
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2322967
Commit-Queue: Andreea Bacanu <aebacanu@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Sreeja Kamishetty <sreejakshetty@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Carlos Caballero <carlscab@google.com>
Cr-Commit-Position: refs/heads/master@{#797607}