Remove a EscapableHandleScope from executeScriptInIsolatedWorld()

It is wrong to call EscapableHandleScope::Escape() multiple times
in executeScriptInIsolatedWorld(). Since the callers of executeScriptInIsolatedWorld()
already have HandleScopes, we can just simply remove a EscapableHandleScope
from executeScriptInIsolatedWorld().

Review URL: https://codereview.chromium.org/713743002

git-svn-id: svn://svn.chromium.org/blink/trunk@185106 bbb929c8-8fbe-4397-9dbb-9b2b20218538