gn: Support build directories outside the source tree.

tools/gn/label_pattern.cc

Index: tools/gn/label_pattern.cc
diff --git a/tools/gn/label_pattern.cc b/tools/gn/label_pattern.cc
index 752898f1acace2f03bdddc25315da680728b6d00..88546b60942c4feb5f96246c7f44f9eb938200bf 100644
--- a/tools/gn/label_pattern.cc
+++ b/tools/gn/label_pattern.cc
@@ -61,7 +61,8 @@ LabelPattern::~LabelPattern() {
 }
 
 // static
-LabelPattern LabelPattern::GetPattern(const SourceDir& current_dir,
+LabelPattern LabelPattern::GetPattern(const base::FilePath& source_root,
+                                      const SourceDir& current_dir,
                                       const Value& value,
                                       Err* err) {
   if (!value.VerifyTypeIs(Value::STRING, err))
@@ -77,7 +78,7 @@ LabelPattern LabelPattern::GetPattern(const SourceDir& current_dir,
   // label resolution code to get all the implicit name stuff.
   size_t star = str.find('*');
   if (star == std::string::npos) {
-    Label label = Label::Resolve(current_dir, Label(), value, err);
+    Label label = Label::Resolve(current_dir, Label(), value, source_root, err);
     if (err->has_error())
       return LabelPattern();
 
@@ -110,7 +111,8 @@ LabelPattern LabelPattern::GetPattern(const SourceDir& current_dir,
     // Parse the inside of the parens as a label for a toolchain.
     Value value_for_toolchain(value.origin(), toolchain_string);
     toolchain_label =
-        Label::Resolve(current_dir, Label(), value_for_toolchain, err);
+        Label::Resolve(current_dir, Label(), value_for_toolchain, source_root,
+                       err);
     if (err->has_error())
       return LabelPattern();
 
@@ -166,7 +168,7 @@ LabelPattern LabelPattern::GetPattern(const SourceDir& current_dir,
     }
 
     // Resolve the non-wildcard stuff.
-    dir = current_dir.ResolveRelativeDir(path);
+    dir = current_dir.ResolveRelativeDir(path, source_root);
     if (dir.is_null()) {
       *err = Err(value, "Label pattern didn't resolve to a dir.",
           "The directory name \"" + path.as_string() + "\" didn't\n"