Web Inspector: speed up _calculateRetainedSizes function

Web Inspector: speed up _calculateRetainedSizes function
https://bugs.webkit.org/show_bug.cgi?id=87863

Patch by Alexei Filippov <alexeif@chromium.org> on 2012-06-04
Reviewed by Yury Semikhatsky.

PerformanceTests:

* inspector/heap-snapshot-advanced.html: Added.
* inspector/heap-snapshot-performance-test.js: Copied from PerformanceTests/inspector/heap-snapshot.html.
(test.performanceTest.step0):
(test.performanceTest.step1):
(test.performanceTest.step2):
(test.performanceTest.cleanup):
(test.performanceTest.done):
(test.performanceTest):
(test):
* inspector/heap-snapshot.html:

Source/WebCore:

* inspector/front-end/HeapSnapshot.js:
(WebInspector.HeapSnapshot.prototype._calculateRetainedSizes):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@119389 268f45cc-cd09-0410-ab3c-d52691b4dbfc