Make WebWorkerFetchContext RefCounted

To share WebWorkerFetchContext from multiple FetchContext.

This CL doesn't change the behavior.

Bug: 880027
Change-Id: I09a8eb8355c5186ffd22456400ae934f586996c2
Reviewed-on: https://chromium-review.googlesource.com/c/1198679
Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606708}
25 files changed