If feedback for call site frequency is 0, then the combined frequency
is still 0, even if the current function invocation count is infinity.

Bug: chromium:919754
Change-Id: I97be096b6b38f934fb13f01b2b22e148c539e1c0
Reviewed-on: https://chromium-review.googlesource.com/c/1404445Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58714}