[gn] Add trace entries for loading and blocking on imports

tools/gn/trace.cc

Index: tools/gn/trace.cc
diff --git a/tools/gn/trace.cc b/tools/gn/trace.cc
index 2a1b460d56b11e856cbc121186a541b4439c35e7..74a3e71c03e73d8bcda5560ff93ad9868a97b839 100644
--- a/tools/gn/trace.cc
+++ b/tools/gn/trace.cc
@@ -170,6 +170,10 @@ void EnableTracing() {
     trace_log = new TraceLog;
 }
 
+bool TracingEnabled() {
+  return !!trace_log;
+}
+
 void AddTrace(TraceItem* item) {
   trace_log->Add(item);
 }
@@ -203,6 +207,8 @@ std::string SummarizeTraces() {
       case TraceItem::TRACE_CHECK_HEADER:
         headers_checked++;
         break;
+      case TraceItem::TRACE_IMPORT_LOAD:
+      case TraceItem::TRACE_IMPORT_BLOCK:
       case TraceItem::TRACE_SETUP:
       case TraceItem::TRACE_FILE_LOAD:
       case TraceItem::TRACE_FILE_WRITE:
@@ -281,6 +287,12 @@ void SaveTraces(const base::FilePath& file_name) {
       case TraceItem::TRACE_FILE_WRITE:
         out << "\"file_write\"";
         break;
+      case TraceItem::TRACE_IMPORT_LOAD:
+        out << "\"import_load\"";
+        break;
+      case TraceItem::TRACE_IMPORT_BLOCK:
+        out << "\"import_block\"";
+        break;
       case TraceItem::TRACE_SCRIPT_EXECUTE:
         out << "\"script_exec\"";
         break;