Move m_shouldOnlyIncludeDirectChildren member from LiveNodeListBase to HTMLCollection

Move m_shouldOnlyIncludeDirectChildren member from LiveNodeListBase to HTMLCollection
as it is only useful to HTMLCollection and not LiveNodeList. This is always false for
LiveNodeList now that ChildNodeList no longer subclasses LiveNodeList after r166565.

A shouldOnlyIncludeDirectChildren() method is added to LiveNodeList which is inlined
and returns false unconditionally. This method is called from templated methods in
LiveNodeListBase and therefore the compiler will be able to optimize the branching
out in the LiveNodeList specializations of these methods.

R=esprehn, adamk

Review URL: https://codereview.chromium.org/136783008

git-svn-id: svn://svn.chromium.org/blink/trunk@166653 bbb929c8-8fbe-4397-9dbb-9b2b20218538