Add staging for skia cl 300707

Skia is moving toward requiring a GrDirectContext when calling
derived image factory methods like makeSubset. This flag tells skia
to retain the current behavior of defaulting to the source image's
context until we update Chrome to the new API.

Change-Id: I5991c7e0860ad02a506c3edf126486b8e136ad25
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2292800
Commit-Queue: Adlai Holler <adlai@google.com>
Auto-Submit: Adlai Holler <adlai@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
Cr-Commit-Position: refs/heads/master@{#787384}

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 https://review.skia.org/300707
+#define SK_IMAGE_SUBSET_USE_SOURCE_CONTEXT
+
 ///////////////////////// Imported from BUILD.gn and skia_common.gypi
 
 /* In some places Skia can use static initializers for global initialization,