Add ShadowUtils sample.

Also enables mouse support in Viewer.

Change-Id: Iaed08d42a64f591f0cd9b24684b3aee43404ed94
Reviewed-on: https://skia-review.googlesource.com/15313
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
diff --git a/gn/samples.gni b/gn/samples.gni
index 7c9f4e2..2397aa2 100644
--- a/gn/samples.gni
+++ b/gn/samples.gni
@@ -82,6 +82,7 @@
   "$_samplecode/SampleRepeatTile.cpp",
   "$_samplecode/SampleShaders.cpp",
   "$_samplecode/SampleShaderText.cpp",
+  "$_samplecode/SampleShadowUtils.cpp",
   "$_samplecode/SampleShip.cpp",
   "$_samplecode/SampleSlides.cpp",
   "$_samplecode/SampleStringArt.cpp",