Clean up app_shell GN build.

extensions/BUILD.gn

Index: extensions/BUILD.gn
diff --git a/extensions/BUILD.gn b/extensions/BUILD.gn
index fe8d83a8b325fe92cc2d03f8f8f72d0a77520d8d..b1e69d27a601735725701718146a837e2170080a 100644
--- a/extensions/BUILD.gn
+++ b/extensions/BUILD.gn
@@ -74,7 +74,7 @@ source_set("test_support") {
   deps = [
     ":extensions_resources",
     "//base",
-    "//content/public/browser",
+    "//base:prefs_test_support",
     "//content/public/common",
     "//content/test:test_support",
     "//extensions/browser",
@@ -87,6 +87,7 @@ source_set("test_support") {
   ]
 
   public_deps = [
+    "//content/public/browser",

[James Cook, 2015/02/13 21:53:31]

Question: Why does this have to be a public_dep?  (I'm not saying it shouldn't,
I just don't understand what motivates it.)

[Ken Rockot(use gerrit already), 2015/02/13 22:08:31]

Good question!

It's because one of the public headers of the test_support target (namely
extensions_test.h) exposes one of the headers of the content/public/browser
target. Without the dependency being public, targets which depend on
test_support may not receive sufficient build configuration to find that header.

That's effectively the only rule to determine choice between deps or
public_deps: if A depends on B and A's public interface exposes this dependency,
then B is a public dependency of A.

     "//extensions/common/api/cast_channel:cast_channel_proto",
   ]