Portals: Fix crash when an orphaned portal navigates.

Bug: 973925
Change-Id: I7eae9ca730babc823341c766e49d242496d2c315
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1672204
Commit-Queue: Lucas Gadani <lfg@chromium.org>
Reviewed-by: Charlie Reis <creis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672118}
3 files changed