gn: Support build directories outside the source tree.

tools/gn/label_unittest.cc

Index: tools/gn/label_unittest.cc
diff --git a/tools/gn/label_unittest.cc b/tools/gn/label_unittest.cc
index 3067ef6f9868bc260727a6889b331d41975f5061..87b7419f0034343e2043bbb7450859568f1621ca 100644
--- a/tools/gn/label_unittest.cc
+++ b/tools/gn/label_unittest.cc
@@ -69,9 +69,10 @@ TEST(Label, Resolve) {
     std::string location, name;
     Err err;
     Value v(NULL, Value::STRING);
+    base::FilePath source_root("/source/root");
     v.string_value() = cur.str;
-    Label result =
-        Label::Resolve(SourceDir(cur.cur_dir), default_toolchain, v, &err);
+    Label result = Label::Resolve(SourceDir(cur.cur_dir), default_toolchain, v,
+                                  source_root, &err);
     EXPECT_EQ(cur.success, !err.has_error()) << i << " " << cur.str;
     if (!err.has_error() && cur.success) {
       EXPECT_EQ(cur.expected_dir, result.dir().value())