Fix Value::GetDouble() crash in SessionDurationUpdater

It looks like occasionally the kObservedSessionTime pref is corrupt or
otherwise incorrect, resulting in a crash upon a Value::GetDouble()
call on a value in the dictionary. This should fix the crash by
verifying the value type.

Bug: 942397
Change-Id: I15389c725dc61d96b12912dbc21ed9e69648852d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1529719
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Collin Baker <collinbaker@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642274}