Fix DCHECK in SessionRestoreStatsCollector.

This was found sometime last year, but has recently been tickled with a
higher frequency due to session restore changes.

BUG=784306

Change-Id: I1d200f94f3fdb4e5fb2989f9f0be11a4dec3606a
Reviewed-on: https://chromium-review.googlesource.com/1138867
Reviewed-by: S├ębastien Marchand <sebmarchand@chromium.org>
Commit-Queue: Chris Hamilton <chrisha@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575741}
1 file changed