Add a UseCounter for CSSCharsetRule.encoding

Add a UseCounter for CSSCharsetRule.encoding

This is by request from Simon Pieters, editor of the CSSOM spec, in
order to investige if getting rid of the CSSCharsetRule interface is
feasible. This has been discussed on blink-dev:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/bWgRZOZKmbk/LZWpPb-ZHJUJ

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=174724

[jochen (gone - plz use gerrit), 2014-05-23 12:55:05]

lgtm

[philipj_slow, 2014-05-23 13:00:52]

The CQ bit was checked by philipj@opera.com

[commit-bot: I haz the power, 2014-05-23 13:01:09]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/philipj@opera.com/299443014/1

[commit-bot: I haz the power, 2014-05-23 19:02:38]

FYI, CQ is re-trying this CL (attempt #1).
Please consider checking whether the failures are real,
and report flakes to chrome-troopers@google.com.
The failing builders are:
  win_blink_rel on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/win_blink_rel/builds/8951)

[commit-bot: I haz the power, 2014-05-23 20:25:51]

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

[commit-bot: I haz the power, 2014-05-23 20:25:51]

Try jobs failed on following builders:
  win_blink_rel on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/win_blink_rel/builds/8972)

[philipj_slow, 2014-05-23 21:45:09]

The CQ bit was checked by philipj@opera.com

[commit-bot: I haz the power, 2014-05-23 21:45:41]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/philipj@opera.com/299443014/1

[commit-bot: I haz the power, 2014-05-23 22:38:23]

Change committed as 174724