Worker: Merge WebWorkerContentSettingsClientProxy into WebContentSettingsClient

As a cleanup, this CL...

 - merges WebWorkerContentSettingsClient into WebContentSettingsClient,
 - renames WebWorkerContentSettingsClientProxy into
   WebWorkerContentSettingsClient, and
 - makes CreateWorkerContentSettingsClient() return std::unique_ptr instead of
   a rawptr w/ ownership.

Bug: 729500
Change-Id: Icc7b9f26f52f3312c16c36b356d2cf9c34b5a99a
Reviewed-on: https://chromium-review.googlesource.com/527763
Reviewed-by: Kentaro Hara <[email protected]>
Reviewed-by: Jochen Eisinger <[email protected]>
Reviewed-by: Kinuko Yasuda <[email protected]>
Commit-Queue: Hiroki Nakagawa <[email protected]>
Cr-Commit-Position: refs/heads/master@{#478614}
26 files changed