Fix inspector-protocol/heap-profiler/heap-snapshot-with-detached-dom-tree.html flakiness

Do additional gc run in attempt to collect all unreachable DOM wrappers that may be left from previous tests.

BUG=312925
R=alph@chromium.org

Review URL: https://codereview.chromium.org/47383003

git-svn-id: svn://svn.chromium.org/blink/trunk@160932 bbb929c8-8fbe-4397-9dbb-9b2b20218538