GN: Replace mojo/system.gni with forwarding groups

mojo/public/gles2/BUILD.gn

Index: mojo/public/gles2/BUILD.gn
diff --git a/mojo/public/gles2/BUILD.gn b/mojo/public/gles2/BUILD.gn
index dc73e2635f3675439b2123c553b95ddec51b6d86..70e19b41fc8b89b92192fe33662b339fa9c18b9a 100644
--- a/mojo/public/gles2/BUILD.gn
+++ b/mojo/public/gles2/BUILD.gn
@@ -41,3 +41,29 @@ static_library("gles2") {
   }
 }
 
+
+# In an is_component_build build, everything can link against //mojo/gles2
+# because it is built as a shared library. However, in a static build,
+# //mojo/gles2 is linked into an executable (e.g., mojo_shell), and must be
+# injected into other shared libraries (i.e., Mojo Apps) that need the mojo
+# gles2 API.
+#
+# For component targets, add //mojo/public/gles2:for_component to your deps
+# section.
+#
+# For shared_library targets (e.g., a Mojo App), add
+# //mojo/public/gles2:for_shared_library to your deps
+
+group("for_shared_library") {
+  if (is_component_build) {
+    deps = [ "//mojo/gles2" ]
+  } else {
+    deps = [ ":gles2" ]
+  }
+}
+
+group("for_component") {
+  if (is_component_build) {
+    deps = [ "//mojo/gles2" ]
+  }
+}