Have HashMap<KeyType, AtomicString>::get() return a const reference

Have HashMap<KeyType, AtomicString>::get() return nullAtom instead of AtomicString()
when the key cannot be found so that the method can now return a const reference.

This avoids useless copies of the AtomicString when calling HashMap::get() when the
value type is AtomicString.

This CL already makes use of this optimization in relevant call sites.

R=tkent

Review URL: https://codereview.chromium.org/102103002

git-svn-id: svn://svn.chromium.org/blink/trunk@163179 bbb929c8-8fbe-4397-9dbb-9b2b20218538