Remove SnapshotTransferHandler

This class has single descendant and used only for tranfering snapshots from the backend to the frontend. It was merged into HeapSnapshotProfileType

BUG=324769
R=loislo@chromium.org

Review URL: https://codereview.chromium.org/135893003

git-svn-id: svn://svn.chromium.org/blink/trunk@166202 bbb929c8-8fbe-4397-9dbb-9b2b20218538