Fix TSAN issue with atlas expansion.

Removes SkString-related malloc from DM crash handler, and adds null check in ProxyRefTest.

Bug: skia:3550
Change-Id: I143c532b5d231a426b1a96b854e1effd6379b673
Reviewed-on: https://skia-review.googlesource.com/48440
Reviewed-by: Mike Klein <mtklein@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
5 files changed