Measure how often Attr needs its associated Element

Measure how often Attr needs its associated Element

The new Attr in http://dom.spec.whatwg.org/#attr does not inherit from
Node and is not attached to an Element. To determine if this is Web
compatible, measure how often the associated Element is actually needed
in the Attr.value getter and setter.

The counters could be affected by Attr.nodeValue and the internal
Attr::childrenChanged(), so measure those too. On the assumption that
they will be very rarely hit compared to Attr.value, don't bother
discriminating whether or not Element was needed.

BUG=none

[philipj_slow, 2014-03-18 17:21:05]

PTAL. There are many ways to get this wrong and I went back and forth a lot
while writing this patch, so please review carefully :)

Note that Attr.ownerElement just came back to the spec today:
https://www.w3.org/Bugs/Public/show_bug.cgi?id=25086

If you don't think that trying to make Attr Element-free is a meaningful
simplification, then this review should just be rejected.

[philipj_slow, 2014-03-18 17:50:38]

There's also some discussion in
https://www.w3.org/Bugs/Public/show_bug.cgi?id=25016

[philipj_slow, 2014-03-18 18:02:16]

Per https://www.w3.org/Bugs/Public/show_bug.cgi?id=25016#c7 I think that this CL
is pointless, I was tricked by a spec bug into thinking the spec was trying to
remove the association. I'll leave it open in case you think having this data
would be interesting, but will close it otherwise.

[abarth-chromium, 2014-03-18 22:50:45]

We can probably just close the CL.