Experiment with blocking script inside fragment-parser-inserted `<iframe srcdoc>`.

Experiment with blocking script inside fragment-parser-inserted `<iframe srcdoc>`.

The discussion at https://github.com/whatwg/html/issues/2300 suggests that
`el.innerHTML = "<script>yay();</script>";` should be considered equivalent
to `el.innerHTML = "<iframe srcdoc='<script>yay();</script>'></iframe>";`.
This patch adds a use counter to measure the impact of such a change, and
adds an experimental flag to test the new behavior.

BUG=684435
R=jochen@chromium.org

[Mike West, 2017-01-27 13:26:25]

The CQ bit was checked by mkwst@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2017-01-27 13:26:43]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[Mike West, 2017-01-27 13:26:51]

WDYT, Jochen?

[commit-bot: I haz the power, 2017-01-27 15:05:30]

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

[commit-bot: I haz the power, 2017-01-27 15:05:31]

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

[jochen (gone - plz use gerrit), 2017-01-27 15:18:07]

lgtm

[Mike West, 2017-01-27 15:49:58]

The CQ bit was checked by mkwst@chromium.org

[Mike West, 2017-01-27 15:49:58]

The patchset sent to the CQ was uploaded after l-g-t-m from jochen@chromium.org
Link to the patchset: https://codereview.chromium.org/2657263002/#ps20001
(title: "Bitwise logic is hard.")

[commit-bot: I haz the power, 2017-01-27 15:50:09]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-01-27 17:48:07]

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

[commit-bot: I haz the power, 2017-01-27 17:48:08]

Try jobs failed on following builders:
  linux_android_rel_ng on master.tryserver.chromium.android (JOB_FAILED,
https://build.chromium.org/p/tryserver.chromium.android/builders/linux_androi...)

[Mike West, 2017-01-31 18:06:24]

The CQ bit was checked by mkwst@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2017-01-31 18:06:59]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-01-31 18:09:17]

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

[commit-bot: I haz the power, 2017-01-31 18:09:19]

Dry run: Try jobs failed on following builders:
  ios-device on master.tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios-device/builds...)
  ios-device-xcode-clang on master.tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios-device-xcode-...)
  ios-simulator on master.tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios-simulator/bui...)
  ios-simulator-xcode-clang on master.tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios-simulator-xco...)