Hash iterators: Check for concurrent modification and fix 1 place where it happened

R=ager@chromium.org, dstockwell@chromium.org, ericwilligers@chromium.org, jochen@chromium.org, mikhail.pozdnyakov@intel.com
BUG=

Review URL: https://codereview.chromium.org/219413002

git-svn-id: svn://svn.chromium.org/blink/trunk@170679 bbb929c8-8fbe-4397-9dbb-9b2b20218538