Make SkBlendMode_AsCoeff() public

Change-Id: Ie704f0f83b1034ff8ee1a5d3cd194e555cae20a3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/252600
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
Auto-Submit: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
diff --git a/RELEASE_NOTES.txt b/RELEASE_NOTES.txt
index e9f78e0..d660a6f 100644
--- a/RELEASE_NOTES.txt
+++ b/RELEASE_NOTES.txt
@@ -17,6 +17,9 @@
   * Remove old async read pixels APIs
     https://review.skia.org/251198
 
+  * Expose SkBlendModeCoeff and SkBlendMode_AsCoeff for Porter-Duff blend modes.
+    https://review.skia.org/252600
+
 * * *
 
 Milestone 79