Fix svg/animations/svglength-element-removed-crash.svg test failure

Apparently calling GCController.collect once is not enough to destroy all dom wrappers that are not used anymore. The change adds additional gc() to make sure all garbage nodes are destroyed.

BUG=307614
R=haraken@chromium.org

Review URL: https://codereview.chromium.org/27483002

git-svn-id: svn://svn.chromium.org/blink/trunk@159724 bbb929c8-8fbe-4397-9dbb-9b2b20218538