Convert the ExtensionApiTest.AutotestPrivate test to use an event page instead of a background page

Convert the ExtensionApiTest.AutotestPrivate test to use an event page instead of a background page

This fixes timeouts caused by flipping the background pages to have throttled DOM timers

R=kalman
BUG=494815
Committed: https://crrev.com/16b8efce9a7b1d671ea89fd147e4e23d267a3786
Cr-Commit-Position: refs/heads/master@{#332688}

[oystein (OOO til 10th of July), 2015-06-03 01:12:02]

one more for the road...

[not at google - send to devlin, 2015-06-03 16:02:40]

lgtm

[oystein (OOO til 10th of July), 2015-06-03 16:09:18]

The CQ bit was checked by oysteine@chromium.org

[commit-bot: I haz the power, 2015-06-03 16:10:38]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1149213005/1

[commit-bot: I haz the power, 2015-06-03 18:10:32]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-06-03 18:10:33]

Try jobs failed on following builders:
  linux_chromium_asan_rel_ng on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[oystein (OOO til 10th of July), 2015-06-03 18:12:09]

The CQ bit was checked by oysteine@chromium.org

[commit-bot: I haz the power, 2015-06-03 18:13:06]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1149213005/1

[commit-bot: I haz the power, 2015-06-03 19:02:55]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-06-03 19:02:56]

Try jobs failed on following builders:
  linux_chromium_asan_rel_ng on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[oystein (OOO til 10th of July), 2015-06-03 20:02:51]

The CQ bit was checked by oysteine@chromium.org

[commit-bot: I haz the power, 2015-06-03 20:04:15]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1149213005/1

[commit-bot: I haz the power, 2015-06-03 21:27:17]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-06-03 21:28:10]

Patchset 1 (id:??) landed as
https://crrev.com/16b8efce9a7b1d671ea89fd147e4e23d267a3786
Cr-Commit-Position: refs/heads/master@{#332688}