[ios] Fix "gn gen" for fat builds

Force use of xctest_bundle_principal_class parameter in the
template ios_xctest_bundle to prevent an error about unused
variable while running "gn gen" while building a fat build.

Bug: none
Change-Id: I4a20d789b1241b51df900e94b0006c6325638296
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1992061
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#730528}
1 file changed