Ensure AtomicString::init() is called

emptyAtom should be a non-null empty string but it is null before
AtomicString::init() is called.

Previously AtomicString::init() is called from CoreInitializer::init(),
but not during platform_blink_unittests causing test KURLTest.Empty to
produce wrong KRUL::isNull() results (and produce correct result if
it's run after FontCache test which calls AtomicString::init()).

Now call AtomicString::init() from WTF::initializeMainThread().
Other callers are removed.

BUG=245441

Review URL: https://codereview.chromium.org/533843002

git-svn-id: svn://svn.chromium.org/blink/trunk@181262 bbb929c8-8fbe-4397-9dbb-9b2b20218538