Fix GrColorMatrixFragmentProcessor handling of premul transparent black.

Use the original alpha in the unpremul color.

Bug: chromium:1024935
Change-Id: I6a721431781f0ef42a3f162d39f8bbac924a2c30
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/261680
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
diff --git a/gn/tests.gni b/gn/tests.gni
index c7acde0..a588c41 100644
--- a/gn/tests.gni
+++ b/gn/tests.gni
@@ -147,6 +147,7 @@
   "$_tests/Matrix44Test.cpp",
   "$_tests/MatrixClipCollapseTest.cpp",
   "$_tests/MatrixTest.cpp",
+  "$_tests/MatrixColorFilterTest.cpp",
   "$_tests/MemoryTest.cpp",
   "$_tests/MemsetTest.cpp",
   "$_tests/MessageBusTest.cpp",