[code cache] Return empty string for secondary code cache keys

- Changes DCHECKs to tests for no prefix or separator. In that case,
  return an empty string for the URL.
- Calling code is changed to test for an empty string before calling
  the URL predicate.

Bug: chromium:1039740
Change-Id: If9e5216ff8e0ed6f952c7cc79bc4bd701ec069b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2057385
Reviewed-by: Mythri Alle <mythria@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#741978}
4 files changed