Avoid duplicating 'form->associatedElements()' call in WebSearchableFormData

Fix FIXME in WebSearchableFormData.

Review URL: https://codereview.chromium.org/87493002

git-svn-id: svn://svn.chromium.org/blink/trunk@162695 bbb929c8-8fbe-4397-9dbb-9b2b20218538