In crrev.com/2856103002 sentinel frequency values were introduced, using
NaN as the sentinel. However the comparison function was not *fully*
updated to support these - comparing two NaNs would give ambiguous
results. This caused test failures when building with VS 2017, probably
because of subtle changes in the arrangement of nodes in the tree.

This change uses the the node ID to break ties. An alternative would be
to use a non-NaN sentinel value.

R=bmeurer@chromium.org
BUG=chromium:722480

Review-Url: https://codereview.chromium.org/2894433004
Cr-Commit-Position: refs/heads/master@{#45415}