[chromedriver] Don't apply device metrics overrides to app and extension pages.

[chromedriver] Don't apply device metrics overrides to app and extension pages.

This prevents a renderer crash, and fixes
MobileEmulationCapabilityTest.testDeviceMetricsWithStandardWidth, which is
currently failing on the Win7 and Mac bots.

BUG=chromedriver:1205
TBR=stgao@chromium.org

Committed: https://crrev.com/d03cc3e31d21b995c2b9cb26edf63ce71b3256fe
Cr-Commit-Position: refs/heads/master@{#345988}

[samuong, 2015-08-27 19:54:29]

The CQ bit was checked by samuong@chromium.org

[commit-bot: I haz the power, 2015-08-27 19:55:04]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1317153002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1317153002/1

[commit-bot: I haz the power, 2015-08-27 20:07:17]

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

[commit-bot: I haz the power, 2015-08-27 20:07:18]

Try jobs failed on following builders:
  win_chromium_x64_rel_ng on tryserver.chromium.win (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.win/builders/win_chromium_x64_...)

[samuong, 2015-08-27 20:45:18]

The CQ bit was checked by samuong@chromium.org

[commit-bot: I haz the power, 2015-08-27 20:45:29]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1317153002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1317153002/20001

[commit-bot: I haz the power, 2015-08-27 21:09:39]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-08-27 21:10:21]

Patchset 2 (id:??) landed as
https://crrev.com/d03cc3e31d21b995c2b9cb26edf63ce71b3256fe
Cr-Commit-Position: refs/heads/master@{#345988}

[stgao, 2015-08-31 18:44:57]

lgtm