Use a single GC instead of multiple GCs on fast/forms/select-set-length-with-mutation-remove.html

r170014 changed multiple GCs to be default, but this test seems to issue GC too much.
This CL modifies the test to call a single GC instead of multiple GCs to avoid taking too much time on the test.
This matches the previous behavior before r170014

BUG=357001

Review URL: https://codereview.chromium.org/214293002

git-svn-id: svn://svn.chromium.org/blink/trunk@170145 bbb929c8-8fbe-4397-9dbb-9b2b20218538