Add the concept of a source set to GN.

tools/gn/secondary/build/config/BUILDCONFIG.gn

Index: tools/gn/secondary/build/config/BUILDCONFIG.gn
diff --git a/tools/gn/secondary/build/config/BUILDCONFIG.gn b/tools/gn/secondary/build/config/BUILDCONFIG.gn
index e48f8968e3d58e60fcce5ebad48d30503668b055..d2e609daca838ba764dfb49cba9a7cd630fc3583 100644
--- a/tools/gn/secondary/build/config/BUILDCONFIG.gn
+++ b/tools/gn/secondary/build/config/BUILDCONFIG.gn
@@ -162,7 +162,7 @@ set_sources_assignment_filter(sources_assignment_filter)
 if (is_component_build) {
   component_mode = "shared_library"
 } else {
-  component_mode = "static_library"
+  component_mode = "source_set"

[scottmg, 2013/10/08 18:42:03]

I don't think this is going to be enough, at least for Windows. We currently
build the |shared_library|s out of a bunch of targets that are the equivalent of
source_sets in the shared_library build and static_library in the static_library
build. It seems like we'd need two "levels" of component?

[brettw, 2013/10/08 20:17:04]

We can continue this on the mailing list. I'm going to check this in since it
seems like we need everything here no matter what.

 }
 
 toolkit_uses_gtk = is_linux
@@ -266,6 +266,10 @@ set_defaults("shared_library") {
   }
 }
 
+set_defaults("source_set") {
+  configs = native_compiler_configs
+}
+
 # ==============================================================================
 # TOOLCHAIN SETUP
 # ==============================================================================