Remove RenderTextControl dependency from HTMLTextFormControlElement.

hasVisibleTextArea took a RenderTextControl argument, but it did nothing
specific to RenderTextControl. RenderObject is enough.

BUG=274205
TEST=none; should have no behavior changes.

Review URL: https://chromiumcodereview.appspot.com/23035006

git-svn-id: svn://svn.chromium.org/blink/trunk@156275 bbb929c8-8fbe-4397-9dbb-9b2b20218538