Revert of Cleanup: Remove extra EmbeddedWorker layering in the renderer SW code ...

Revert of Cleanup: Remove extra EmbeddedWorker layering in the renderer SW code (patchset #5 id:140001 of https://codereview.chromium.org/1136323003/)

Reason for revert:
* WebString should not be directly passed to base::Bind for cross-thread posting
* Caused crash in a layout test: https://crbug.com/492625

Original issue's description:
> Cleanup: Remove extra EmbeddedWorker layering in the renderer SW code
>
> Mechanical changes only:
> - Combine ServiceWorkerScriptContext and EmbeddedWorkerContextClient,
>   renaming it to ServiceWorkerContextClient
> - Rename EmbeddedWorkerContextMessageFilter to ServiceWorkerContextMessageFilter
>
> ServiceWorker/EmbeddedWorker separation in the renderer side
> is currently awkwardly done and require a lot of extra methods
> forwarding between the two classes, looking at the current status
> probably we could combine them.
>
> This leaves all IPC messages and browser-side code as is, at least
> for now.
>
> We could do similar cleanups for those areas too, but currently
> I'm not really motivated to do further cleanups as they don't
> seem causing much extra overhead in our productivity etc.
> (Code-health wise we could probably do better, if anyone's
> motivated to make further cleanups they're welcome)
>
> BUG=491039
> TEST=existing tests
>
> Committed: https://crrev.com/29882b428437d2c70c2950c4699dc9277f7daa61
> Cr-Commit-Position: refs/heads/master@{#331533}

TBR=falken@chromium.org,serviceworker-reviews@chromium.org,avi@chromium.org,hiroshige@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=491039

Review URL: https://codereview.chromium.org/1159933002

Cr-Commit-Position: refs/heads/master@{#331567}