Avoid compilation error on older compilers

The constness on null_provider causes:
 "default initialization of an object of const type
 'const NullResourceProvider' without a user-provided default constructor"

See https://stackoverflow.com/a/47368753 for more.

TL;DR; clang 3.8 and older is not happy

Bug: skia:
Change-Id: Icfc38680163c3bd4952c0e35551706cad8dbfee6
Reviewed-on: https://skia-review.googlesource.com/131521
Commit-Queue: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Florin Malita <fmalita@chromium.org>
1 file changed