add a config for custom skcms defines

This won't have any effect until the CL that makes SKCMS_LEGACY_POWF
relevant lands: https://skia-review.googlesource.com/c/skcms/+/128041

Change-Id: If890bc620e726288422184876fb50275a7e0082d
Reviewed-on: https://chromium-review.googlesource.com/1057871Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558392}

skia/BUILD.gn

@@ -557,6 +557,10 @@ component("skia") {
   sources += [ "//third_party/skia/src/core/SkColorSpaceXform_skcms.cpp" ]
 }
 
+config("chromium_specific_config_for_skcms") {
+  defines = [ "SKCMS_LEGACY_POWF" ]
+}
+
 # Template for things that are logically part of :skia, but need to be split out
 # so custom compile flags can be applied.
 #