Add the concept of a source set to GN.

tools/gn/secondary/chrome/BUILD.gn

Index: tools/gn/secondary/chrome/BUILD.gn
diff --git a/tools/gn/secondary/chrome/BUILD.gn b/tools/gn/secondary/chrome/BUILD.gn
index 133bf8214ce0e820c9003797066082bdf84810c0..4e6df28a6bc547cd692a6e426811aa786e136bc1 100644
--- a/tools/gn/secondary/chrome/BUILD.gn
+++ b/tools/gn/secondary/chrome/BUILD.gn
@@ -96,7 +96,7 @@ static_library("common") {
 }
 
 # TODO(brettw) move to browser/devtools/BUILD.gn
-static_library("debugger") {
+source_set("debugger") {
   sources = [
     "browser/devtools/adb/android_rsa.cc",
     "browser/devtools/adb/android_rsa.h",
@@ -167,15 +167,15 @@ static_library("debugger") {
 
 }
 
-static_library("plugin") {
+source_set("plugin") {
   external = true
 }
 
-static_library("renderer") {
+source_set("renderer") {
   external = true
 }
 
-static_library("utility") {
+source_set("utility") {
   external = true
 }