Set m_obj as private member variable

To fix one of "FIXME" comment, this cl move m_obj from public to private in InlineIterator.h.

BUG=N/A

Review URL: https://codereview.chromium.org/82563002

git-svn-id: svn://svn.chromium.org/blink/trunk@162518 bbb929c8-8fbe-4397-9dbb-9b2b20218538