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>
diff --git a/modules/skottie/src/Skottie.cpp b/modules/skottie/src/Skottie.cpp
index d2c2cce..93c71e9 100644
--- a/modules/skottie/src/Skottie.cpp
+++ b/modules/skottie/src/Skottie.cpp
@@ -1231,7 +1231,7 @@
         std::unique_ptr<SkStream> openStream(const char[]) const { return nullptr; }
     };
 
-    const NullResourceProvider null_provider;
+    NullResourceProvider null_provider;
     const auto anim = sk_sp<Animation>(new Animation(provider ? *provider : null_provider,
                                                      std::move(version), size, fps, json, stats));
     const auto t2 = SkTime::GetMSecs();