Avoid using instanceRoot in layout tests

Avoid using instanceRoot in layout tests, instead get the correspong element directly by using getElementById.

BUG=313438

Review URL: https://codereview.chromium.org/278033003

git-svn-id: svn://svn.chromium.org/blink/trunk@173827 bbb929c8-8fbe-4397-9dbb-9b2b20218538