Drop unnecessary RadioNodeList::m_onlyMatchImgElements member

Drop unnecessary RadioNodeList::m_onlyMatchImgElements member and replace it by
an inline function that does a CollectionType comparison.

The generated assembly for RadioNodeList.o is about the same size (1 line
shorter).

R=adamk@chromium.org

Review URL: https://codereview.chromium.org/480473002

git-svn-id: svn://svn.chromium.org/blink/trunk@180374 bbb929c8-8fbe-4397-9dbb-9b2b20218538