Stop calling [de]ref in ScriptPromiseResolverWithContext destructor.

The ScriptPromiseResolverWithContext destructor is checking if it is misused,
but the current code calls ref() and deref(), which are  invalid in the destructor.
This CL replaces the implementation with a simple assertion.

BUG=386479

Review URL: https://codereview.chromium.org/344733004

git-svn-id: svn://svn.chromium.org/blink/trunk@176518 bbb929c8-8fbe-4397-9dbb-9b2b20218538