Display average time between collections

Display average time between collections

BUG=
R=koda@google.com

Committed: https://code.google.com/p/dart/source/detail?r=37749

[koda, 2014-06-26 17:49:39]

Did you consider using 'uptime' from the VM object to do the division on the
observatory side?

[Cutch, 2014-06-26 17:56:03]

On 2014/06/26 17:49:39, koda wrote:
> Did you consider using 'uptime' from the VM object to do the division on the
> observatory side?

We want to measure the time from the time the isolate (not the VM) was created.
This corresponds to the time that the heaps were created.

I did wonder about including an isolate up time but then we get into a situation
where we need to refresh the isolate and the heap information in order to
compute this property.

[koda, 2014-06-26 19:42:38]

LGTM

https://codereview.chromium.org/353003005/diff/1/runtime/vm/pages.cc
File runtime/vm/pages.cc (right):

https://codereview.chromium.org/353003005/diff/1/runtime/vm/pages.cc#newcode389
runtime/vm/pages.cc:389: int64_t run_time = OS::GetCurrentTimeMicros() -
isolate->start_time();
Note to self: yet another reason to consider a Space base class.

[Cutch, 2014-06-26 19:53:39]

Committed patchset #2 manually as r37749 (presubmit successful).