Remove ASSERT(map.size() == isolatedWorldCount) from DOMWrapperWorld.cpp

Now that we have a private script isolated world, map.size() is not guaranteed to be equal to isolatedWorldCount(). We can just remove the ASSERT (rather than inventing a more complicated ASSERT condition).

BUG=394230

Review URL: https://codereview.chromium.org/404473003

git-svn-id: svn://svn.chromium.org/blink/trunk@178362 bbb929c8-8fbe-4397-9dbb-9b2b20218538