Add SkOverdrawColorFilter

Uses the value in the src alpha channel to choose how
to set the dst pixel.

This is a part of a multi-part change to detect and
display gpu overdraw on Android.

CQ_INCLUDE_TRYBOTS=master.client.skia.compile:Build-Ubuntu-GCC-x86_64-Debug-NoGPU-Trybot

BUG:32370375

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=5113

Change-Id: I07040929d8a46bbadd499dccec75eebef0e11d11
Reviewed-on: https://skia-review.googlesource.com/5113
Commit-Queue: Matt Sarett <msarett@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
diff --git a/gn/gm.gni b/gn/gm.gni
index 7e70c89..4732e03 100644
--- a/gn/gm.gni
+++ b/gn/gm.gni
@@ -198,6 +198,7 @@
   "$_gm/occludedrrectblur.cpp",
   "$_gm/offsetimagefilter.cpp",
   "$_gm/ovals.cpp",
+  "$_gm/overdrawcolorfilter.cpp",
   "$_gm/OverStroke.cpp",
   "$_gm/patch.cpp",
   "$_gm/patchgrid.cpp",