Use m_targetElementInstance in fewer places

Use m_targetElementInstance in fewer places since SVGElementInstance
is scheduled for removal.

BUG=313438

Review URL: https://codereview.chromium.org/269843002

git-svn-id: svn://svn.chromium.org/blink/trunk@173273 bbb929c8-8fbe-4397-9dbb-9b2b20218538