Allow form inputs to be potential cluster roots.

We check for form inputs in blockSuppressesAutosizing, but the block
must be a potential cluster root to reach this check.

BUG=376012

Review URL: https://codereview.chromium.org/334333002

git-svn-id: svn://svn.chromium.org/blink/trunk@176251 bbb929c8-8fbe-4397-9dbb-9b2b20218538