Add optional public header checking to GN build

tools/gn/trace.cc

Index: tools/gn/trace.cc
diff --git a/tools/gn/trace.cc b/tools/gn/trace.cc
index a2f34a32d6624d3d4666823568fd4145d4b65f0b..acd735df365cef7707c9cf18c3efc6e29d5d1328 100644
--- a/tools/gn/trace.cc
+++ b/tools/gn/trace.cc
@@ -185,6 +185,8 @@ std::string SummarizeTraces() {
   std::vector<const TraceItem*> parses;
   std::vector<const TraceItem*> file_execs;
   std::vector<const TraceItem*> script_execs;
+  std::vector<const TraceItem*> check_headers;
+  int headers_checked = 0;
   for (size_t i = 0; i < events.size(); i++) {
     switch (events[i]->type()) {
       case TraceItem::TRACE_FILE_PARSE:
@@ -196,6 +198,12 @@ std::string SummarizeTraces() {
       case TraceItem::TRACE_SCRIPT_EXECUTE:
         script_execs.push_back(events[i]);
         break;
+      case TraceItem::TRACE_CHECK_HEADERS:
+        check_headers.push_back(events[i]);
+        break;
+      case TraceItem::TRACE_CHECK_HEADER:
+        headers_checked++;
+        break;
       case TraceItem::TRACE_FILE_LOAD:
       case TraceItem::TRACE_FILE_WRITE:
       case TraceItem::TRACE_DEFINE_TARGET:
@@ -211,6 +219,19 @@ std::string SummarizeTraces() {
   SummarizeScriptExecs(script_execs, out);
   out << std::endl;
 
+  // Generally there will only be one header check, but it's theoretically
+  // possible for more than one to run if more than one build is going in
+  // parallel. Just report the total of all of them.
+  if (!check_headers.empty()) {
+    float check_headers_time = 0;
+    for (size_t i = 0; i < check_headers.size(); i++)
+      check_headers_time += check_headers[i]->delta().InMillisecondsF();
+
+    out << "Header check time: (total time in ms, files checked)\n";
+    out << base::StringPrintf(" %8.2f  %d\n",
+                              check_headers_time, headers_checked);
+  }
+
   return out.str();
 }
 
@@ -261,6 +282,13 @@ void SaveTraces(const base::FilePath& file_name) {
         break;
       case TraceItem::TRACE_DEFINE_TARGET:
         out << "\"define\"";
+        break;
+      case TraceItem::TRACE_CHECK_HEADER:
+        out << "\"hdr\"";
+        break;
+      case TraceItem::TRACE_CHECK_HEADERS:
+        out << "\"header_check\"";
+        break;
     }
 
     if (!item.toolchain().empty() || !item.cmdline().empty()) {