Fix TODO in chrome_web_view, use utils.lookup to find context menu events.

Fix TODO in chrome_web_view, use utils.lookup to find context menu events.
This was done previously accessing array by index, which is brittle b/c
this depends on the ordering of the events in web_view_internal.json.

BUG=None
Test=None, internal only cleanup.

Committed: https://crrev.com/9a3fb195ffb589e011803aa17610aeff4a696683
Cr-Commit-Position: refs/heads/master@{#322588}

[lazyboy, 2015-03-26 23:33:36]

lazyboy@chromium.org changed reviewers:
+ paulmeyer@chromium.org

[paulmeyer, 2015-03-27 13:04:45]

lgtm

[lazyboy, 2015-03-27 16:09:59]

The CQ bit was checked by lazyboy@chromium.org

[commit-bot: I haz the power, 2015-03-27 16:10:24]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1039043003/20001

[commit-bot: I haz the power, 2015-03-27 16:11:41]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-03-27 16:13:11]

Patchset 2 (id:??) landed as
https://crrev.com/9a3fb195ffb589e011803aa17610aeff4a696683
Cr-Commit-Position: refs/heads/master@{#322588}