pass storage directly to allocator for blitters

BUG=skia:

Change-Id: If93df0437b2aba813042241f00eb0c6fba2a3c6f
Reviewed-on: https://skia-review.googlesource.com/9161
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Reed <reed@google.com>

src/core/SkDraw.cpp

@@ -73,7 +73,8 @@ private:
     // Owned by fAlloc, which will handle the delete.
     SkBlitter*          fBlitter;
 
-    SkArenaAlloc fAlloc{kSkBlitterContextSize};
+    char fStorage[kSkBlitterContextSize];
+    SkArenaAlloc fAlloc{fStorage};
 };
 #define SkAutoBlitterChoose(...) SK_REQUIRE_LOCAL_VAR(SkAutoBlitterChoose)