Unflake fast/loader/resources/unload-hyperlink-targeted-subframe.html

Prior to this change --iterations=10 would fail about 50%
of the runs of this test.  After this change I see 0 failures.

Unclear if this test still tests exactly what it set out to
but it seems like it should.

R=japhet@chromium.org

Review URL: https://codereview.chromium.org/318523002

git-svn-id: svn://svn.chromium.org/blink/trunk@175416 bbb929c8-8fbe-4397-9dbb-9b2b20218538