Add a use counter API

Add a use counter API

This lets embedders track certain features of v8 and the number of times
they are used

BUG=none
R=svenpanne@chromium.org, marja@chromium.org
LOG=y

Committed: https://code.google.com/p/v8/source/detail?r=21925

[Sven Panne, 2014-06-23 09:29:39]

https://codereview.chromium.org/346233002/diff/1/include/v8.h
File include/v8.h (right):

https://codereview.chromium.org/346233002/diff/1/include/v8.h#newcode4132
include/v8.h:4132: * Features reported via the SetUseCounterCallback callback.
Do not chang
+e

https://codereview.chromium.org/346233002/diff/1/include/v8.h#newcode4136
include/v8.h:4136: enum UseCounterFeature {
I general, I'm a bit torn how to represent such a bag of random, ad-hoc,
sure-to-be-changed-later things:

   * One could use an enum like this plus a comment like the one above, but at
some point one will reach the state when some enums are not supported anymore,
new ones have to be introduced, etc.

   * One could go the more untyped way of using e.g. strings, which have less
static guarantees, but lead to less API churn.

There are well-known existing API examples in the real world for both variants,
so I really don't know what's better here, let's discuss this offline...

https://codereview.chromium.org/346233002/diff/1/src/isolate.cc
File src/isolate.cc (right):

https://codereview.chromium.org/346233002/diff/1/src/isolate.cc#newcode2353
src/isolate.cc:2353: if (use_counter_callback_)
Following the good example of the Heartbleed bug here? :-)

[jochen (gone - plz use gerrit), 2014-06-23 09:37:29]

updated, ptal

[Sven Panne, 2014-06-23 09:37:36]

LGTM, Chrome uses a similar enum.

[jochen (gone - plz use gerrit), 2014-06-23 09:47:07]

Committed patchset #2 manually as r21925 (presubmit successful).