Fix inspector/profiler/heap-snapshot-comparison-sorting.html flakiness

The test failed when running after inspector-protocol/heap-profiler/heap-snapshot-with-detached-dom-tree.html because the latter left a snapshot on the backend and no existing snapshots were expected in the former tests.

BUG=305029
R=loislo@chromium.org

Review URL: https://codereview.chromium.org/26999002

git-svn-id: svn://svn.chromium.org/blink/trunk@159440 bbb929c8-8fbe-4397-9dbb-9b2b20218538