Add a macro for querying the byte order of SkPMColor.

include/core/SkPostConfig.h

Index: include/core/SkPostConfig.h
===================================================================
--- include/core/SkPostConfig.h	(revision 8061)
+++ include/core/SkPostConfig.h	(working copy)
@@ -187,6 +187,28 @@
     #endif
 #endif
 
+/**
+ * SK_PMCOLOR_BYTE_ORDER can be used to query the byte order of SkPMColor at compile time. The
+ * relationship between the byte order and shift values depends on machine endianness. If the shift
+ * order is R=0, G=8, B=16, A=24 then ((char*)&pmcolor)[0] will produce the R channel on a little
+ * endian machine and the A channel on a big endian machine. Thus, given those shifts values,
+ * SK_PMCOLOR_BYTE_ORDER(R,G,B,A) will be true on a little endian machine and
+ * SK_PMCOLOR_BYTE_ORDER(A,B,G,R) will be true on a big endian machine.
+ */
+#ifdef SK_CPU_BENDIAN
+    #define SK_PMCOLOR_BYTE_ORDER(C0, C1, C2, C3)     \
+        (SK_ ## C3 ## 32_SHIFT == 0  &&             \
+         SK_ ## C2 ## 32_SHIFT == 8  &&             \
+         SK_ ## C1 ## 32_SHIFT == 16 &&             \
+         SK_ ## C0 ## 32_SHIFT == 24)
+#else
+    #define SK_PMCOLOR_BYTE_ORDER(C0, C1, C2, C3)     \
+        (SK_ ## C0 ## 32_SHIFT == 0  &&             \
+         SK_ ## C1 ## 32_SHIFT == 8  &&             \
+         SK_ ## C2 ## 32_SHIFT == 16 &&             \
+         SK_ ## C3 ## 32_SHIFT == 24)
+#endif
+
 //  stdlib macros
 
 #if 0