FinishCompilationUnits used the assumption that FinishCompilationUnit
only return null if there is no compilation unit left to be finished.
This assumption was wrong though, because also a compilation error can
cause the result to be null. Therefore I switched to use the function
index as a new indicator.

BUG=chromium:709174

Change-Id: I3e9689fd71b8364422e1c74404921df2799191aa
Reviewed-on: https://chromium-review.googlesource.com/471347
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44468}