Remove usages of HeapProfiler.finishHeapSnapshot from pyautolib

HeapProfiler.finishHeapSnapshot is being removed from the protocol (see https://codereview.chromium.org/27000005/). This change makes RemoteInspectorClient use the same mechanism as in DevTools front-end.

BUG=306475
R=dennisjeffrey@chromium.org, marja@chromium.org
TBR=kkania@chromium.org

Review URL: https://codereview.chromium.org/27018002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228650 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228650