Improve GN public header file checking

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 00197e679b3a4d7fa3098b65fc31746a56508697..673c865fe255d3a79f0e50598f52af57f0380354 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -32,7 +32,6 @@ group("root") {
     #"//sdch",
     #"//skia",
     #"//third_party/WebKit/Source/platform",
-    "//third_party/freetype2",
     #"//third_party/icu:icudata",
     #"//third_party/leveldatabase",
     "//skia",
@@ -48,4 +47,10 @@ group("root") {
     "//ui/gfx/geometry",
     "//url",
   ]
+
+  if (is_linux) {
+    deps += [
+      "//third_party/freetype2",
+    ]
+  }
 }