QUIC - Record the FailureReason for WaitForDataReady in
Net.QuicDiskCache.FailureReason.WaitForDataReady histogram
From Start() until WaitForDataReady or it's callbacks are
completed, we save the FailureReason in a |last_failure_|.
When WaitForDataReady or its callbacks finish, we record
last_failure_ if there is a failure.
Renamed user_callback_ to wait_for_ready_callback_.
R=rch@chromium.org, asvitkine@chromium.org
BUG=432961
Review URL: https://codereview.chromium.org/
726433002
Cr-Commit-Position: refs/heads/master@{#304082}