[ios] Prevent placeholder crash

A previous fix did not fully fix the crash. https://crrev.com/c/1645479

The crash with the signature AddSameConstraints still occur so an
additional check is added directly before AddSameConstraints() is

Bug: 970876
Change-Id: I9adb9437eea159896d089ce415c33688fb1dc46b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1670149
Commit-Queue: edchin <edchin@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672115}
1 file changed