gn: Support build directories outside the source tree.

tools/gn/substitution_writer.cc

Index: tools/gn/substitution_writer.cc
diff --git a/tools/gn/substitution_writer.cc b/tools/gn/substitution_writer.cc
index 65a1d1fe2859f23388922df978ddb2cab1a71d48..aab01f80d1b0a8b713e7bf550cd8dc0090cc7c68 100644
--- a/tools/gn/substitution_writer.cc
+++ b/tools/gn/substitution_writer.cc
@@ -235,10 +235,6 @@ OutputFile SubstitutionWriter::ApplyPatternToSourceAsOutputFile(
     const SubstitutionPattern& pattern,
     const SourceFile& source) {
   SourceFile result_as_source = ApplyPatternToSource(settings, pattern, source);
-  CHECK(result_as_source.is_source_absolute())
-      << "The result of the pattern \""
-      << pattern.AsString()
-      << "\" was not an absolute path beginning in \"//\".";
   return OutputFile(settings->build_settings(), result_as_source);
 }
 
@@ -358,7 +354,8 @@ std::string SubstitutionWriter::GetSourceSubstitution(
       if (source.is_system_absolute())
         return DirectoryWithNoLastSlash(source.GetDir());
       return RebaseSourceAbsolutePath(
-          DirectoryWithNoLastSlash(source.GetDir()), SourceDir("//"));
+          DirectoryWithNoLastSlash(source.GetDir()), SourceDir("//"),
+          settings->build_settings()->root_path());
 
     case SUBSTITUTION_SOURCE_GEN_DIR:
       to_rebase = DirectoryWithNoLastSlash(
@@ -382,7 +379,8 @@ std::string SubstitutionWriter::GetSourceSubstitution(
   // extension extraction) will have been handled via early return above.
   if (output_style == OUTPUT_ABSOLUTE)
     return to_rebase;
-  return RebaseSourceAbsolutePath(to_rebase, relative_to);
+  return RebaseSourceAbsolutePath(to_rebase, relative_to,
+                                  settings->build_settings()->root_path());
 }
 
 // static