gn: Support build directories outside the source tree.

tools/gn/source_file.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "tools/gn/source_file.h"
 
 #include "base/logging.h"
 #include "build/build_config.h"
 #include "tools/gn/filesystem_utils.h"
 #include "tools/gn/source_dir.h"
 
+namespace {
+
+void AssertValueSourceFileString(const std::string& s) {
+#if defined(OS_WIN)
+  DCHECK(s[0] == '/' ||
+         (s.size() > 2 && s[0] != '/' && s[1] == ':' && IsSlash(s[2])));
+#else
+  DCHECK(s[0] == '/');
+#endif
+  DCHECK(!EndsWithSlash(s));
+}
+
+}  // namespace
+
 SourceFile::SourceFile() {
 }
 
 SourceFile::SourceFile(const base::StringPiece& p)
     : value_(p.data(), p.size()) {
   DCHECK(!value_.empty());
-  DCHECK(value_[0] == '/');
-  DCHECK(!EndsWithSlash(value_));
+  AssertValueSourceFileString(value_);
 }
 
 SourceFile::SourceFile(SwapIn, std::string* value) {
   value_.swap(*value);
   DCHECK(!value_.empty());
-  DCHECK(value_[0] == '/');
-  DCHECK(!EndsWithSlash(value_));
+  AssertValueSourceFileString(value_);
 }
 
 SourceFile::~SourceFile() {
 }
 
 std::string SourceFile::GetName() const {
   if (is_null())
     return std::string();
 
   DCHECK(value_.find('/') != std::string::npos);
@@ skipping 25 matching lines @@
     }
     return base::FilePath(UTF8ToFilePath(converted));
   }
 
   converted.assign(&value_[2], value_.size() - 2);
   if (source_root.empty())
     return UTF8ToFilePath(converted).NormalizePathSeparatorsTo('/');
   return source_root.Append(UTF8ToFilePath(converted))
       .NormalizePathSeparatorsTo('/');
 }