Remove redundant m_isItemCacheValid flag from LiveNodeListBase

We can simply do a null-check on the m_cachedItem pointer to check
if the cache is valid.

R=eseidel, esprehn
BUG=None

Review URL: https://codereview.chromium.org/136833002

git-svn-id: svn://svn.chromium.org/blink/trunk@165261 bbb929c8-8fbe-4397-9dbb-9b2b20218538