Simplify isolates access during stack iteration (WAS: Move SafeStackFrameIterator::active_count_...)

While trying to fix Mac and Windows versions for this change:
http://codereview.chromium.org/6771047/, I figured out, that we
already store an isolate in StackFrameIterator, so we can use it in
frame objects, instead of requiring it from caller.

I've changed iterators usage to the following scheme: whenever a
caller maintains an isolate pointer, it just passes it to stack
iterator, and no more worries about passing it to frame content
accessors.  If a caller uses current isolate, it can omit passing it
to iterator, in this case, an iterator will use the current isolate,
too.

There was a special case with LiveEdit, which creates
detached copies of frame objects.

R=vitalyr@chromium.org
BUG=none
TEST=none

Review URL: http://codereview.chromium.org/6794019

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7499 ce2b1a6d-e550-0410-aec6-3dcde31c8c00