work around Flutter/Fuchsia GN problems

Flutter used to depend on GN subtargets, and while it
doesn't any more, the version in Fuchsia still does.

Change-Id: I6afb95ae6c353bcc78b42fefd37dcdebe76c4aa1
Reviewed-on: https://skia-review.googlesource.com/c/170340
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index f23716b..ee55eaa 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -560,7 +560,7 @@
   }
 }
 
-optional("gpu") {
+optional("gpu_for_real") {
   enabled = skia_enable_gpu
   deps = [
     ":compile_processors",
@@ -820,12 +820,19 @@
   }
 }
 
+# Temporary fake targets so Flutter-in-Fuchsia can "depend" on them.
+# TODO: rename :gpu_for_real back to :gpu when cleaning up.
+source_set("effects") {
+}
+source_set("gpu") {
+}
+
 component("skia") {
   public_configs = [ ":skia_public" ]
   configs += skia_library_configs
 
   public_deps = [
-    ":gpu",
+    ":gpu_for_real",
     ":pdf",
     ":skcms",
   ]