Add an idle migration period to QuicChromiumClientSession.

Sessions will only be migrated if its idle period is within this period.

Bug: 929351
Change-Id: I18b919f679ee8ed06b40b832685a0e10b9c02a77
Reviewed-on: https://chromium-review.googlesource.com/c/1459072
Commit-Queue: Zhongyi Shi <zhongyi@chromium.org>
Reviewed-by: Ryan Hamilton <rch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631010}
15 files changed