Make document.elementFromPoint() to work with OPTION elements

Since r159012, which makes document.caretRangeFromPoint() to work with :first-letter style by getting text offsets, document.elementFromPoint() uses RenderObject from HitTestResult rather than HitTestResult.innerNode(). This works fine for all cases except for node which doesn't have RenderObject, e.g. OPTION element.

This patch restores behavior before r159012 for document.elementFromPoint().
Note: :first-letter style doesn't affect result of document.elementFromPoint(), because it doesn't use text offsets.

BUG=335010
TEST=LayoutTests/fast/dom/Document/document-elementFromPoint-on-option-element.html

Review URL: https://codereview.chromium.org/137313007

git-svn-id: svn://svn.chromium.org/blink/trunk@165426 bbb929c8-8fbe-4397-9dbb-9b2b20218538