Don't update PageState for a SiteInstance mismatch.

BUG=766262
TEST=See bug for repro.

Change-Id: Ifb087b687acd40d8963ef436c9ea82ca2d960117
Reviewed-on: https://chromium-review.googlesource.com/674808
Commit-Queue: Charlie Reis (OOO until 9/25) <creis@chromium.org>
Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/master@{#503297}