Remove unneccessary RELEASE_ASSERTs.

The same check (in fact, a more accurate one) is taken care of in
StringImpl::{createUninitialized,reallocate}

Review URL: https://chromiumcodereview.appspot.com/22281003

git-svn-id: svn://svn.chromium.org/blink/trunk@155680 bbb929c8-8fbe-4397-9dbb-9b2b20218538