Add a UseCounter for HTMLElement.webkitdropzone

Add a UseCounter for HTMLElement.webkitdropzone

The unprefixed attribute is not supported yet, but knowing the usage of
the prefixed attribute is still valuable.

R=jochen@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=178839

[jochen (gone - plz use gerrit), 2014-07-24 11:55:47]

lgtm

[philipj_slow, 2014-07-24 12:02:33]

The CQ bit was checked by philipj@opera.com

[commit-bot: I haz the power, 2014-07-24 12:03:31]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/philipj@opera.com/415093002/1

[commit-bot: I haz the power, 2014-07-24 12:21:48]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  win_blink_rel on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/win_blink_rel/builds/18715)

[commit-bot: I haz the power, 2014-07-24 13:26:16]

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

[commit-bot: I haz the power, 2014-07-24 13:26:16]

Try jobs failed on following builders:
  win_blink_rel on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/win_blink_rel/builds/18725)

[philipj_slow, 2014-07-24 13:53:41]

Committed patchset #1 manually as r178839 (presubmit successful).