Add null check to signin view code to handle new error page isolation.

This is a speculative crash fix, since there are no repro steps for
a test.

BUG=860409

Change-Id: I10b5668635fd18f996245f8eb24b553cdf315eb0
Reviewed-on: https://chromium-review.googlesource.com/1128386
Reviewed-by: Mihai Sardarescu <[email protected]>
Commit-Queue: Charlie Reis <[email protected]>
Cr-Commit-Position: refs/heads/master@{#573818}
1 file changed