Content Shell: Introduce LayoutTestBrowserContext. (Reland)

Content Shell: Introduce LayoutTestBrowserContext. (Reland)

This splits the DRT-specific parts of ShellBrowserContext out into a new
class that we'll only instantiate when the flag is set.

------------------------------------------------------------------------
This is a re-land of https://codereview.chromium.org/637843003, which
was reverted due to layout test timeouts.
------------------------------------------------------------------------

BUG=420994
Committed: https://crrev.com/d993d06a69153686d5a99c07d7825c5557a7ffd0
Cr-Commit-Position: refs/heads/master@{#299092}

[Mike West, 2014-10-10 10:41:17]

mkwst@chromium.org changed reviewers:
+ vabr@chromium.org

[Mike West, 2014-10-10 10:41:18]

Vaclav pointed out to me that calling a virtual function from a constructor is a
Bad Idea. Since the thing I was trying to was only one line, and it doesn't
matter if it happens before or after the initialization function runs, I've just
inlined the assignment into the subclass' constructor.

Mind taking a look at the diff between patchset 1 and 2, Vaclav?

-mike

[Mike West, 2014-10-10 11:16:57]

mkwst@chromium.org changed reviewers:
+ apavlov@chromium.org

[Mike West, 2014-10-10 11:16:57]

apavlov: FYI, I'll probably be relanding this shortly. The blink bots are happy,
so I hope it won't break things again. :)

[vabr (Chromium), 2014-10-10 11:23:43]

The diff LGTM.
Thanks!
Vaclav

[Mike West, 2014-10-10 11:25:04]

The CQ bit was checked by mkwst@chromium.org

[commit-bot: I haz the power, 2014-10-10 11:25:21]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/641343002/160001

[commit-bot: I haz the power, 2014-10-10 11:44:46]

Committed patchset #2 (id:160001)

[commit-bot: I haz the power, 2014-10-10 11:45:32]

Patchset 2 (id:??) landed as
https://crrev.com/d993d06a69153686d5a99c07d7825c5557a7ffd0
Cr-Commit-Position: refs/heads/master@{#299092}