This is a simple refactoring to make the code dealing with the marker
symbols for the default resolve/reject handlers more readable.

Bug: v8:7253
Change-Id: Ife279e77007d52ad11fed3726009d60d7c352c33
Reviewed-on: https://chromium-review.googlesource.com/850696Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50366}