Remove bogus CHECK in set_isolate.
Remove bogus CHECK in set_isolate.

This was added in 22da76be1ee64120f69ed35d6f21c85c31df08b6
but breaks all navigation (which is sadly only tested by our
demo site).

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1182543002.

chinmaygarde/mojo,jianglu/mojo,chinmaygarde/mojo,jianglu/mojo,jianglu/mojo,chinmaygarde/mojo,chinmaygarde/mojo,chinmaygarde/mojo,afandria/mojo,chinmaygarde/mojo,jianglu/mojo,jianglu/mojo,chinmaygarde/mojo,afandria/mojo,jianglu/mojo,afandria/mojo,afandria/mojo,chinmaygarde/mojo,afandria/mojo,afandria/mojo,jianglu/mojo,jianglu/mojo,afandria/mojo,afandria/mojo,chinmaygarde/mojo