Add unit test for pinned SkImages

Change-Id: I2e14353bc865b5994cc90ad643a6a58604955957
Reviewed-on: https://skia-review.googlesource.com/53500
Reviewed-by: Derek Sollenberger <djsollen@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
diff --git a/gn/tests.gni b/gn/tests.gni
index 389d876..34c10b5 100644
--- a/gn/tests.gni
+++ b/gn/tests.gni
@@ -160,6 +160,7 @@
   "$_tests/PictureBBHTest.cpp",
   "$_tests/PictureShaderTest.cpp",
   "$_tests/PictureTest.cpp",
+  "$_tests/PinnedImageTest.cpp",
   "$_tests/PipeTest.cpp",
   "$_tests/PixelRefTest.cpp",
   "$_tests/Point3Test.cpp",