GN: is_skia_standalone

is_skia_standalone will be defined and set to true iff we're using our
standalone BUILDCONFIG.gn.  Any other build (Fuchsia, Chrome, Pdfium)
will have their own BUILDCONFIG.gn, which presumably does not define
is_skia_standalone.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2357963002

Review-Url: https://codereview.chromium.org/2357963002
diff --git a/BUILD.gn b/BUILD.gn
index dddf118..be7a975 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -5,6 +5,10 @@
 
 import("gn/shared_sources.gni")
 
+if (!defined(is_skia_standalone)) {
+  is_skia_standalone = false
+}
+
 declare_args() {
   skia_use_expat = true
   skia_use_fontconfig = is_linux
@@ -20,10 +24,8 @@
 
   skia_enable_android_framework_defines = false
   skia_enable_gpu = true
-  skia_enable_tools = !is_fuchsia
-
-  # TODO: Vulkan debug layers should be disabled for all client uses of skia as well.
-  skia_enable_vulkan_debug_layers = !is_fuchsia && is_debug
+  skia_enable_tools = is_skia_standalone
+  skia_enable_vulkan_debug_layers = is_skia_standalone && is_debug
 }
 
 # Our tools require static linking (they use non-exported symbols) and GPU support (just lazy).