Add GN output prefix override and allow empty output extensions.

tools/gn/binary_target_generator.cc

Index: tools/gn/binary_target_generator.cc
diff --git a/tools/gn/binary_target_generator.cc b/tools/gn/binary_target_generator.cc
index 923d0edd7a95f37ac66538e62a67cefcf68aac52..7c81232dacc2de445f9451bed52a953d84cd3964 100644
--- a/tools/gn/binary_target_generator.cc
+++ b/tools/gn/binary_target_generator.cc
@@ -31,6 +31,9 @@ void BinaryTargetGenerator::DoRun() {
   if (!FillOutputName())
     return;
 
+  if (!FillOutputPrefixOverride())
+    return;
+
   if (!FillOutputExtension())
     return;
 
@@ -85,6 +88,16 @@ bool BinaryTargetGenerator::FillOutputName() {
   return true;
 }
 
+bool BinaryTargetGenerator::FillOutputPrefixOverride() {
+  const Value* value = scope_->GetValue(variables::kOutputPrefixOverride, true);
+  if (!value)
+    return true;
+  if (!value->VerifyTypeIs(Value::BOOLEAN, err_))
+    return false;
+  target_->set_output_prefix_override(value->boolean_value());
+  return true;
+}
+
 bool BinaryTargetGenerator::FillOutputExtension() {
   const Value* value = scope_->GetValue(variables::kOutputExtension, true);
   if (!value)