Fix reporting of simultaneous uncaught exceptions in workers

Static flag isReportingException in messageHandlerInWorker was replaced with a per isolate one. The test was failing consisitently on my machine before the fix and now it passes.

BUG=425960

Review URL: https://codereview.chromium.org/684763003

git-svn-id: svn://svn.chromium.org/blink/trunk@184532 bbb929c8-8fbe-4397-9dbb-9b2b20218538