Since the CallDAG is needed to check for the call graph depth, its
creation code must be robust to very deep call graphs.

BUG=angleproject:1517

Change-Id: I753ab84ad4ced6363a212f0fe94a89aabf4e3c3e
Reviewed-on: https://chromium-review.googlesource.com/385496Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>