Prevent creation of zero-sized proxies

This wasn't seen locally b.c. it is an assert and I only ran release locally and the CQ also only runs release.

I have added linux_trusty_blink_dbg as a try job.

TBR=bsalomon@google.com
Bug: 715392
Change-Id: I010626cb97e886d2fbfd767f948bc640f0534338
Reviewed-on: https://skia-review.googlesource.com/14361
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
3 files changed