gn: Support build directories outside the source tree.

tools/gn/source_dir.cc

Index: tools/gn/source_dir.cc
diff --git a/tools/gn/source_dir.cc b/tools/gn/source_dir.cc
index c59cf88cd61136bc4e1fa6db7620b8c55e736736..97247ffd90ae23cbd12c91f334ec011e47377347 100644
--- a/tools/gn/source_dir.cc
+++ b/tools/gn/source_dir.cc
@@ -41,7 +41,7 @@ SourceDir::~SourceDir() {
 
 SourceFile SourceDir::ResolveRelativeFile(
     const base::StringPiece& p,
-    const base::StringPiece& source_root) const {
+    const base::FilePath& source_root) const {
   SourceFile ret;
 
   // It's an error to resolve an empty string or one that is a directory
@@ -56,7 +56,8 @@ SourceFile SourceDir::ResolveRelativeFile(
     return ret;
   } else if (IsPathAbsolute(p)) {
     if (source_root.empty() ||
-        !MakeAbsolutePathRelativeIfPossible(source_root, p, &ret.value_)) {
+        !MakeAbsolutePathRelativeIfPossible(source_root.value(), p,
+                                            &ret.value_)) {
 #if defined(OS_WIN)
       // On Windows we'll accept "C:\foo" as an absolute path, which we want
       // to convert to "/C:..." here.
@@ -69,17 +70,28 @@ SourceFile SourceDir::ResolveRelativeFile(
     return ret;
   }
 
+  if (!source_root.empty()) {
+    std::string absolute = Resolve(source_root).Append(p.as_string()).value();
+    NormalizePath(&absolute);
+    if (!MakeAbsolutePathRelativeIfPossible(source_root.value(), absolute,
+                                            &ret.value_))
+      ret.value_ = absolute;
+    return ret;
+  }
+
+  // With no source_root_, there's nothing we can do about
+  // e.g. p=../../../path/to/file and value_=//source and we'll
+  // errornously return //file.
   ret.value_.reserve(value_.size() + p.size());
   ret.value_.assign(value_);
   ret.value_.append(p.data(), p.size());
-
   NormalizePath(&ret.value_);
   return ret;
 }
 
 SourceDir SourceDir::ResolveRelativeDir(
     const base::StringPiece& p,
-    const base::StringPiece& source_root) const {
+    const base::FilePath& source_root) const {
   SourceDir ret;
 
   if (p.empty())
@@ -93,7 +105,8 @@ SourceDir SourceDir::ResolveRelativeDir(
     return ret;
   } else if (IsPathAbsolute(p)) {
     if (source_root.empty() ||
-        !MakeAbsolutePathRelativeIfPossible(source_root, p, &ret.value_)) {
+        !MakeAbsolutePathRelativeIfPossible(source_root.value(), p,
+                                            &ret.value_)) {
 #if defined(OS_WIN)
       if (p[0] != '/')  // See the file case for why we do this check.
         ret.value_ = "/";
@@ -106,6 +119,17 @@ SourceDir SourceDir::ResolveRelativeDir(
     return ret;
   }
 
+  if (!source_root.empty()) {
+    std::string absolute = Resolve(source_root).Append(p.as_string()).value();
+    NormalizePath(&absolute);
+    if (!MakeAbsolutePathRelativeIfPossible(source_root.value(), absolute,
+                                            &ret.value_))
+      ret.value_ = absolute;
+    if (!EndsWithSlash(ret.value_))
+      ret.value_.push_back('/');
+    return ret;
+  }
+
   ret.value_.reserve(value_.size() + p.size());
   ret.value_.assign(value_);
   ret.value_.append(p.data(), p.size());