Cache Storage: Remove unused Service/Embedder Worker plumbing

Follow up to https://codereview.chromium.org/1029423004 that cleans up
the now-unused plumbing for the Cache Storage API routed through
Service Worker and Embedded Worker classes. Service Workers and other
contexts can access the API via the embedder API.

BUG=439389
R=kinuko@chromium.org,michaeln@chromium.org

Review URL: https://codereview.chromium.org/1022703003

Cr-Commit-Position: refs/heads/master@{#322386}
4 files changed