Add constant for blitter context size.

src/core/SkBitmapProcShader.h

Index: src/core/SkBitmapProcShader.h
diff --git a/src/core/SkBitmapProcShader.h b/src/core/SkBitmapProcShader.h
index e2d3900a17a6db64af7f4904217cb0f09c1eebda..4df2614ca408db95b8afeb0a0ae59f6c9d7f02c0 100644
--- a/src/core/SkBitmapProcShader.h
+++ b/src/core/SkBitmapProcShader.h
@@ -50,12 +50,14 @@ private:
     typedef SkShader INHERITED;
 };
 
+enum {kSkBlitterContextSize = 2400};
+
 // Commonly used allocator. It currently is only used to allocate up to 3 objects. The total
 // bytes requested is calculated using one of our large shaders, its context size plus the size of
 // an Sk3DBlitter in SkDraw.cpp
 // Note that some contexts may contain other contexts (e.g. for compose shaders), but we've not
 // yet found a situation where the size below isn't big enough.
-typedef SkSmallAllocator<3, 2400> SkTBlitterAllocator;
+typedef SkSmallAllocator<3, kSkBlitterContextSize> SkTBlitterAllocator;
 
 // If alloc is non-nullptr, it will be used to allocate the returned SkShader, and MUST outlive
 // the SkShader.