Remove V8TryCatchRethrowScope from private script bindings

When a private script throws an exception, what the private script binding does is:

- if the exception is the one supported in private scripts,
the private script binding clones the exception in the user's script and throws it.

- if the exception is the one not (yet) supported in private scripts,
the private script binding dies.

Since these operations are done explicitly, we should use ReThrow()
instead of relying on V8TryCatchRethrowScope. (V8TryCatchRethrowScope is not
a proper mechanism for doing this because V8TryCatchRethrowScope is intended
to be used to rethrow an exception the way it is without doing any conversion.)

BUG=341031

Review URL: https://codereview.chromium.org/461593002

git-svn-id: svn://svn.chromium.org/blink/trunk@179955 bbb929c8-8fbe-4397-9dbb-9b2b20218538