added support for push_constant layout

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2187433003

Committed: https://skia.googlesource.com/skia/+/fa5f65ac61fd525029aa9dab161ffe4896c10f6d
Review-Url: https://codereview.chromium.org/2187433003
diff --git a/gn/tests.gni b/gn/tests.gni
index 2cf9d33..98980ec 100644
--- a/gn/tests.gni
+++ b/gn/tests.gni
@@ -204,6 +204,7 @@
   "$_tests/SkSharedMutexTest.cpp",
   "$_tests/SkSLErrorTest.cpp",
   "$_tests/SkSLGLSLTest.cpp",
+  "$_tests/SkSLMemoryLayoutTest.cpp",
   "$_tests/SmallAllocatorTest.cpp",
   "$_tests/SortTest.cpp",
   "$_tests/SpecialImageTest.cpp",