Make SVG leak tests less flaky by better utilizing numberOfLiveNodes

The old test was measuring the starting point before several yields to
the event loop, leading to flakiness if some task in flight happened
to free some pre-existing Nodes from another test.

This patch instead measures internals.numberOfLiveNodes just before
and just after removing the nodes-to-be-measured from the DOM.

BUG=307325, 307614
R=ojan@chromium.org, pdr@chromium.org

Review URL: https://codereview.chromium.org/27621002

git-svn-id: svn://svn.chromium.org/blink/trunk@159900 bbb929c8-8fbe-4397-9dbb-9b2b20218538