Revert of Content Shell: Introduce LayoutTestBrowserContext.

Revert of Content Shell: Introduce LayoutTestBrowserContext. (patchset #2 id:20001 of https://codereview.chromium.org/637843003/)

Reason for revert:
Tentatively resulted in multiple layout test timeouts across the chromium.webkit waterfall.

Original issue's description:
> Content Shell: Introduce LayoutTestBrowserContext.
>
> This splits the DRT-specific parts of ShellBrowserContext out into a new
> class that we'll only instantiate when the flag is set.
>
> BUG=420994
>
> Committed: https://crrev.com/98ce5ad6bdd2cd058c21f19b997032f3b958cb73
> Cr-Commit-Position: refs/heads/master@{#299068}

TBR=jochen@chromium.org,mkwst@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=420994
Committed: https://crrev.com/297746b9c09c220f55f470cc02caaee9495ae61c
Cr-Commit-Position: refs/heads/master@{#299077}

[apavlov, 2014-10-10 08:23:24]

Created Revert of Content Shell: Introduce LayoutTestBrowserContext.

[apavlov, 2014-10-10 08:23:36]

The CQ bit was unchecked by apavlov@chromium.org

[apavlov, 2014-10-10 08:23:51]

The CQ bit was checked by apavlov@chromium.org

[commit-bot: I haz the power, 2014-10-10 08:24:57]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/641203004/1

[commit-bot: I haz the power, 2014-10-10 08:25:33]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-10-10 08:26:21]

Patchset 1 (id:??) landed as
https://crrev.com/297746b9c09c220f55f470cc02caaee9495ae61c
Cr-Commit-Position: refs/heads/master@{#299077}

[Mike West, 2014-10-10 10:47:04]

LGTM. I've confirmed that this patch broke layout tests, and I'm working on a
replacement in https://codereview.chromium.org/641343002/. Thanks for the timely
revert, apavlov!