Web Inspector: native memory snapshot performance and coverage test implementation.

Web Inspector: native memory snapshot performance and coverage test implementation.
https://bugs.webkit.org/show_bug.cgi?id=89363

We have to track the performance of MemoryAgent.getProcessMemoryDistribution.
Also this test will work as a burn down chart for Unknown memory metric.

Reviewed by Pavel Feldman.

* inspector/native-memory-snapshot.html: Added.
* inspector/performance-test.js:
(initialize_TimeTracker.InspectorTest.runPerformanceTest.Timer):
(initialize_TimeTracker.InspectorTest.runPerformanceTest.Timer.prototype.reportSize):
(initialize_TimeTracker.InspectorTest.runPerformanceTest.Timer.prototype._dump):
(initialize_TimeTracker.InspectorTest.runPerformanceTest):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@120618 268f45cc-cd09-0410-ab3c-d52691b4dbfc