[FCM] UMA histogram for how long it take to read persisted GCM messages

When a message arrives for a lazy GCM subscription and Chrome isn't in
the foreground, it is persisted on disk until next time Chrome is in the
foreground.

This CL adds two histograms
1- Record the time spent to check if a subscription lazy upon receiving
a GCM message while Chrome is in the background.
2- Record the time spent in reading and replaying persisted
messages when Chrome goes to the foreground.

Bug: 882887
Change-Id: I2ea8e5a5c5242b573e842b56f08d7f1070d48856
Reviewed-on: https://chromium-review.googlesource.com/c/1333656
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Peter Conn <peconn@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Commit-Queue: Mohamed Amir Yosef <mamir@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608925}