[ios][web] Check for GURL validity before using GURL::spec()

GURL::spec() crashes when called on an invalid GURL, so check for
validity of the object before calling GURL::spec() as part of the
serialization to protobuf message.

Fixed: 1496150, 1495992
Change-Id: I53faedbb1aef8b64cc053c1d3edded2912475fb1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4989446
Reviewed-by: Ali Juma <ajuma@chromium.org>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1217033}