gn: Support build directories outside the source tree.

tools/gn/label_pattern_unittest.cc

Index: tools/gn/label_pattern_unittest.cc
diff --git a/tools/gn/label_pattern_unittest.cc b/tools/gn/label_pattern_unittest.cc
index 21d6a9b12149f39cea41864b06e8ec1f697f1b74..be890277a048f3425a4c44a56b0a01e68b978462 100644
--- a/tools/gn/label_pattern_unittest.cc
+++ b/tools/gn/label_pattern_unittest.cc
@@ -22,6 +22,7 @@ struct PatternCase {
 }  // namespace
 
 TEST(LabelPattern, PatternParse) {
+  base::FilePath source_root("/source/root");
   SourceDir current_dir("//foo/");
   PatternCase cases[] = {
     // Missing stuff.
@@ -60,8 +61,9 @@ TEST(LabelPattern, PatternParse) {
   for (size_t i = 0; i < arraysize(cases); i++) {
     const PatternCase& cur = cases[i];
     Err err;
-    LabelPattern result =
-        LabelPattern::GetPattern(current_dir, Value(NULL, cur.input), &err);
+    LabelPattern result = LabelPattern::GetPattern(source_root, current_dir,
+                                                   Value(NULL, cur.input),
+                                                   &err);
 
     EXPECT_EQ(cur.success, !err.has_error()) << i << " " << cur.input;
     EXPECT_EQ(cur.type, result.type()) << i << " " << cur.input;