Simple clean-up of PrivateScriptRunner.cpp

- Use WTF_LOG_ERROR instead of FATAL (because FATAL crashes immediately and we cannot get subsequent error messages).
- Rename installClass() to installClassIfNeeded().
- Add a HandleScope to installClassIfNeeded().

BUG=341031

Review URL: https://codereview.chromium.org/393243002

git-svn-id: svn://svn.chromium.org/blink/trunk@178257 bbb929c8-8fbe-4397-9dbb-9b2b20218538