We can use the script type to determine that instead. Script of type
TYPE_NATIVE are considered builtins, TYPE_NORMAL are not. The only exception
to this rule is the empty function, for which the script is TYPE_NATIVE
(observable by the debugger), but should be stringified to "function () {}"
instead of "function () { [native code] }". For this, I introduce a
hide_source flag on the script object.

We also use IsBuiltin and IsSubjectToDebugging interchangeably. For debugger,
we now use the latter, hiding the detail that only non-builtins are debuggable.

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/1292283004

Cr-Commit-Position: refs/heads/master@{#30285}