Correctly attribute exceptions thrown inside a Worker's imported scripts.

Currently, we have strange behavior when a script loaded into a worker
via 'importScripts()' throws an exception:

* Rather than reporting the exception's full details for same-origin
  scripts, the exception is rethrown from the point at which
  'importScripts()' is called. This makes it difficult to determine
  where the exception actually occurred.

* Exceptions thrown from cross-origin scripts are incorrectly
  sanitized: the message is generally "Uncaught undefined", and the
  file/line/column all point to the 'importScripts()' call.

This patch corrects both cases: cross-origin exceptions are sanitized
in the same way that we sanitize non-worker exceptions (message of
"Script error.", blank file, line and column both set to 0). Same-origin
exceptions now contain accurate file/line/column information.

Note that this patch does not address the 'error' parameter for
same-origin exceptions.

BUG=269538

Review URL: https://chromiumcodereview.appspot.com/22467005

git-svn-id: svn://svn.chromium.org/blink/trunk@155824 bbb929c8-8fbe-4397-9dbb-9b2b20218538