Do not store heap snapshots on the backend

Several HeapProfiler commands and events that required uid to identify snapshot stored on the devtools backend have been removed. The snapshot is automatically deleted right after it has been sent to the front-end.

BUG=324769
R=alph@chromium.org, pfeldman@chromium.org

Review URL: https://codereview.chromium.org/149583002

git-svn-id: svn://svn.chromium.org/blink/trunk@166136 bbb929c8-8fbe-4397-9dbb-9b2b20218538