Revert of Revert of Converted some extension browser tests into using event pages

Revert of Revert of Converted some extension browser tests into using event pages (patchset #1 id:1 of https://codereview.chromium.org/1159593004/)

Reason for revert:
Reverting the revert, the test failure was actually fixed by this revert: https://codereview.chromium.org/1159353003

Original issue's description:
> Revert of Converted some extension browser tests into using event pages (patchset #1 id:60001 of https://codereview.chromium.org/1108133002/)
>
> Reason for revert:
> browser_tests ExtensionApiTest.AutotestPrivate failed on MSan bot
>
> BUG=494815
>
> Original issue's description:
> > Converted some extension browser tests into using event pages, to fix some timeout issues when background pages have their DOM timers throttled.
> >
> > R=kalman
> > BUG=
> >
> > Committed: https://crrev.com/1dc63af0201f5429133b814be52aab24f8d71f16
> > Cr-Commit-Position: refs/heads/master@{#332059}
>
> TBR=kalman@chromium.org,oysteine@chromium.org
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=
>
> Committed: https://crrev.com/cda8c938f06f9955ac895099d05a9db3b61f3ab5
> Cr-Commit-Position: refs/heads/master@{#332159}

TBR=kalman@chromium.org,zhaoqin@google.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=494815
Committed: https://crrev.com/542018467975906306ea2ba5355996011f5f947f
Cr-Commit-Position: refs/heads/master@{#332519}

[oystein (OOO til 10th of July), 2015-06-03 01:06:52]

Created Revert of Revert of Converted some extension browser tests into using
event pages

[commit-bot: I haz the power, 2015-06-03 01:08:34]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1155333003/1

[commit-bot: I haz the power, 2015-06-03 01:11:38]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-06-03 01:12:28]

Patchset 1 (id:??) landed as
https://crrev.com/542018467975906306ea2ba5355996011f5f947f
Cr-Commit-Position: refs/heads/master@{#332519}