Split nodes and edges into separate arrays in heap profiler.
This allowed the following changes:
- heap profiler now makes one pass less over the heap.
- HeapEntriesMap does not allocate EntryInfo per each entry.
- there's no need for an extra pass to set indexes before serialization.
As a result snapshot taking time has reduced up to 2x times.
Review URL: https://chromiumcodereview.appspot.com/
10353010
git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@11531
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