Fix UMA stat for expiration of certificate memory decisions

In our certificate memory decisions experiment, an UMA stat has been
incorrectly recording when a decision is made after expiration for users
in the "forget decisions at session end" group. As a fix, this CL adds a
GUID to represent the current browsing session to the
ChromeSSLHostStateDelegate and records that identifier whenever a
decision is made. Then, in a future session, Chrome can tell if the
decision has expired if the GUID doesn't match the current session's
GUID.

A good question to ask might be, "why use a GUID to represent the
browsing session rather than simply iterating over all the current
entries and marking them as expired?" Well, I'm glad you asked!
Unfortunately, content settings does not allow one to iterate over the
values of *compound* content settings; it only works for simply content
settings, which certificate decisions most certainly are not. While this
could be added, it would be a fair amount work for what amounts to a
temporary problem, as this will all go away once the experiment is
finished.

BUG=418631

Review URL: https://codereview.chromium.org/610063002

Cr-Commit-Position: refs/heads/master@{#297671}

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