[oilpan]: Avoid refcounting QualifiedName's nullQName when tracing.
    
When doing tracing across threads I hit an assert due to tracing the SMILTimeContainer's hashmap from a different thread than the one it was allocated on. Basically in HashTable's trace method we call isEmptyOrDeletedBucket which ends up calling PairHashTraits::emptyValue which makes a pair<WeakMember<SVGElement>, QualifiedName> initialized with the individual empty values. This causes the nullQName to be copied to the new pair which causes a ref() in RefCountedBase hitting ASSERT(m_verifier.isSafeToUse()).

With this change all global or static local QualifiedNames are ignoring ref counting. This is similar to the way StringImpl works.

R=ager@chromium.org, erik.corry@gmail.com, haraken@chromium.org, oilpan-reviews@chromium.org, tkent@chromium.org, vegorov@chromium.org, zerny@chromium.org
BUG=

Review URL: https://codereview.chromium.org/311803003

git-svn-id: svn://svn.chromium.org/blink/trunk@176307 bbb929c8-8fbe-4397-9dbb-9b2b20218538