[AltSvc] Save expiration as int64 string instead of double.

[AltSvc] Save expiration as int64 string instead of double.

Save alternative service expiration times as int64 in a string instead of
double in HttpServerPropertiesManager::SaveAlternativeServiceToServerPrefs(),
and load expiration time accordingly in
HttpServerPropertiesManager::ParseAlternativeServiceDict().  The reason is that
numeric_limits<int64>::max() can be serialized in JSON, whereas
numeric_limits<double>::max() cannot.  That is, this change allows
base::Time::Max() expiration times.  Note that JSON does not support int64, so
it has to be converted into a string.

This change might result in unexpected behavior on user installations where
alternative services were persisted to disk as double with earlier versions.
However, as https://crrev.com/1227063002 has not even made it to Beta yet,
this can be safely ignored.

BUG=516034

Review URL: https://codereview.chromium.org/1275663002

Cr-Commit-Position: refs/heads/master@{#343209}