Update InspectorBackend.TakeJSHeapSnapshot to make it work with the new HeapProfiler behavior

HeapProfiler.getHeapSnapshot command is being removed from the protocol. Heap snapshot is now sent in response to HeapProfiler.takeHeapSnapshot command.

BUG=324769

Review URL: https://codereview.chromium.org/141553008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248161 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248161