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

Added additional gc() to the test to make sure all detached DOM trees that may be left since the previous tests are collected.

BUG=239549
R=alph@chromium.org

Review URL: https://codereview.chromium.org/47923003

git-svn-id: svn://svn.chromium.org/blink/trunk@160718 bbb929c8-8fbe-4397-9dbb-9b2b20218538