Support viewing/changing backend shaders in Viewer

Previously, all backends allowed the SkSL to be edited, and GL allowed
GLSL to be edited. Now any backend's source can be seen, and they can
all be edited (other than SPIR-V). Tested with HLSL and SPIRV. I don't
have a Mac available, but MSL should work, too.

Change-Id: Ia2a11bb5922dd49a5f25840e48384e0246a28b69
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/296856
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index d5cf02b..bc79e29 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -2360,6 +2360,9 @@
         deps += [ ":experimental_xform" ]
         sources += [ "gm/xform.cpp" ]
       }
+      if (skia_use_vulkan) {
+        deps += [ "//third_party/spirv-tools:spvtools" ]
+      }
     }
   }