move SK_ENABLE_DISCRETE_GPU to cover only tools

Change-Id: I390bd2cfae16651cf109c91b1047caf235cd6df4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/210033
Commit-Queue: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 2c6c1cf..657208f 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -198,9 +198,6 @@
   if (skia_use_angle) {
     defines += [ "SK_ANGLE" ]
   }
-  if (skia_enable_discrete_gpu) {
-    defines += [ "SK_ENABLE_DISCRETE_GPU" ]
-  }
   if (!is_official_build) {
     defines += [ "GR_TEST_UTILS=1" ]
   }
@@ -1491,6 +1488,11 @@
     public_defines = []
     public_include_dirs += [ "tools/gpu" ]
 
+    defines = []
+    if (skia_enable_discrete_gpu) {
+      defines += [ "SK_ENABLE_DISCRETE_GPU" ]
+    }
+
     deps = []
     public_deps = []
     sources = [