Make sure there is no text output when converting from PDF to PS for
text with size 0.

BUG=767343,827748

Change-Id: Id788b1f936d4f97b2201b7171a756a7ec2892bf1
Reviewed-on: https://chromium-review.googlesource.com/989299
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547779}