Add flag to stage Skia API change

See (future) Skia CL: https://skia-review.googlesource.com/c/skia/+/298509

Bug: skia:10425
Change-Id: Ie090d0cdcc4f6aeee1a845cecf699f4e2f7bd617
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2261229
Auto-Submit: Adlai Holler <adlai@google.com>
Commit-Queue: Adlai Holler <adlai@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Florin Malita <fmalita@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781988}

skia/config/SkUserConfig.h

@@ -217,6 +217,9 @@ SK_API void SkDebugf_FileLine(const char* file,
 // Staging for lowp::bilerp_clamp_8888, and for planned misc. others.
 #define SK_DISABLE_LOWP_BILERP_CLAMP_CLAMP_STAGE
 
+// Staging for migrating SkDeferredDisplayList from unique_ptr to sk_sp.
+#define SK_DDL_IS_UNIQUE_POINTER
+
 ///////////////////////// Imported from BUILD.gn and skia_common.gypi
 
 /* In some places Skia can use static initializers for global initialization,