Add UseCounters for CSS-related *List interfaces with item() methods

Add UseCounters for CSS-related *List interfaces with item() methods

By request from Simon Pieters (CSSOM editor) measure the usage of the
item() method on the three interfaces that are currently marked as
[ArrayClass]. This is to determine the feasibility of removing the usage
of [ArrayClass] by using "real" arrays instead. That won't be possible
in the case of MediaList, because that interface has other members, but
measure it anyway in case some option emerges where removing
MediaList.item() would in some way help.

Also add a UseCounter for StyleSheetList's anonymous named getter, which
is the only other thing blocking array conversion of that interface.

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=194040

[philipj_slow, 2015-04-20 14:58:00]

philipj@opera.com changed reviewers:
+ ed@opera.com

[philipj_slow, 2015-04-20 14:58:00]

PTAL

[Erik Dahlström (inactive), 2015-04-20 15:13:12]

Not an owner, but LGTM.

Consider if it's worth adding a testcase to check that the counters count the
right thing, though I do trust this to work.

[philipj_slow, 2015-04-20 15:20:37]

On 2015/04/20 15:13:12, Erik Dahlström wrote:
> Not an owner, but LGTM.
> 
> Consider if it's worth adding a testcase to check that the counters count the
> right thing, though I do trust this to work.

Thanks! The generated ones (V8_*) can be trusted to work, and the explicit one
ought to work assuming the code path is taken. Unfortunately it's only by
deprecating that one can test this via LayoutTests, so if any of these don't
work it'll had a few months to the process, but no big deal.

[philipj_slow, 2015-04-20 15:20:45]

The CQ bit was checked by philipj@opera.com

[commit-bot: I haz the power, 2015-04-20 15:21:27]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1072403008/1

[commit-bot: I haz the power, 2015-04-20 17:39:40]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=194040