Remove WebDocument::createEvent.

Remove WebDocument::createEvent.

We shouldn't expose the type unsafe API for creating events, instead lets
expose constructors for the actual event objects. I also removed most of
the functionality of WebDOMCustomEvent since it's only used by the IPCEcho
testing system. As this system has no tests checked in and seems to be only
used by local benchmarking we should probably remove it, but lets do that
in a future patch.

Committed: https://crrev.com/4aae4de1c3375d1f216dc0316c272414c5dd9b12
Cr-Commit-Position: refs/heads/master@{#351135}

[esprehn, 2015-09-26 03:36:42]

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

[commit-bot: I haz the power, 2015-09-26 03:36:48]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1367333002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1367333002/1

[esprehn, 2015-09-26 03:37:12]

esprehn@chromium.org changed reviewers:
+ dglazkov@chromium.org, jochen@chromium.org, pdr@chromium.org

[commit-bot: I haz the power, 2015-09-26 05:33:00]

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

[commit-bot: I haz the power, 2015-09-26 05:33:01]

Dry run: This issue passed the CQ dry run.

[jochen (gone - plz use gerrit), 2015-09-28 11:30:15]

lgtm

[esprehn, 2015-09-28 17:22:36]

The CQ bit was checked by esprehn@chromium.org

[commit-bot: I haz the power, 2015-09-28 17:23:55]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1367333002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1367333002/1

[commit-bot: I haz the power, 2015-09-28 20:53:38]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-28 20:54:57]

Patchset 1 (id:??) landed as
https://crrev.com/4aae4de1c3375d1f216dc0316c272414c5dd9b12
Cr-Commit-Position: refs/heads/master@{#351135}