The CodeStubAssembler::SubString method should not try to be defensive
about non-string inputs, but really enforce String receiver. Passing
a non-String input will result in crash in the %SubString runtime
function anyway.

R=jgruber@chromium.org

Review-Url: https://codereview.chromium.org/2700103002
Cr-Commit-Position: refs/heads/master@{#43267}