Add additional usage metrics for DOMStringList methods

Add additional usage metrics for DOMStringList methods

We'd like to remove DOMStringList from the web platform in favor of
plain old arrays of strings. Usage of the contains() method is low but
non-zero, and usage may differ between sources of DOMStringList -
Indexed DB vs. Location.ancestorOrigins. Add measurement for the use
of the item() method (vs. indexed getter), and for both contains() and
item() distinguish between sources of instances.

BUG=460726
R=philipj@opera.com

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=197783

[jsbell, 2015-06-24 19:38:35]

I factored the data-gathering part out of
https://codereview.chromium.org/966843002/ since actual deprecation warnings
don't seem to be getting the love on blink-dev. Also, updating the enum
constants when I rebase is annoying. :)

philipj@ - please take a look?

[philipj_slow, 2015-06-24 22:18:10]

The CQ bit was checked by philipj@opera.com

[philipj_slow, 2015-06-24 22:18:10]

lgtm

Yay

[commit-bot: I haz the power, 2015-06-24 22:18:34]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1207793004/1

[commit-bot: I haz the power, 2015-06-24 22:22:24]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=197783