Don't crash for subframes after in-page navigations in --site-per-process.

Don't crash for subframes after in-page navigations in --site-per-process.

This is a stopgap until we can clone the subframe FNEs from the last
committed entry on in-page navigations.  See bug comment 5 for details.

BUG=522193
TEST=See bug comment 4 for repro steps.

Committed: https://crrev.com/e062d54a2a6f42af08653ac5ae201028b7be0ac1
Cr-Commit-Position: refs/heads/master@{#345238}

[Charlie Reis, 2015-08-24 21:00:04]

creis@chromium.org changed reviewers:
+ avi@chromium.org

[Charlie Reis, 2015-08-24 21:11:49]

Avi, can you take a look?  My larger plans for cloning the FNEs are described in
comment 5 of the bug.  This should avoid crashing in the meantime.

[Avi (use Gerrit), 2015-08-25 00:42:28]

lgtm

[Charlie Reis, 2015-08-25 01:57:11]

The CQ bit was checked by creis@chromium.org

[commit-bot: I haz the power, 2015-08-25 01:57:28]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1303333006/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1303333006/1

[commit-bot: I haz the power, 2015-08-25 02:02:15]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-25 02:03:07]

Patchset 1 (id:??) landed as
https://crrev.com/e062d54a2a6f42af08653ac5ae201028b7be0ac1
Cr-Commit-Position: refs/heads/master@{#345238}