Add --trace-rotate-startup-file option and store unsaved trace data on exit.

content/browser/tracing/add_new_file_index.cc

+#include "content/browser/tracing/add_new_file_index.h"
+
+#include "base/file_util.h"
+#include "base/strings/string_number_conversions.h"
+#include "base/threading/thread_restrictions.h"
+
+namespace {
+const unsigned int kMaxLogFileIndex = 1000000;
+}
+
+namespace content {
+
+base::FilePath AddNewLogFileIndex(const base::FilePath& trace_file) {
+  // trace file should be initialized synchronously
+  base::ThreadRestrictions::ScopedAllowIO allow_io;
+  for (unsigned int i = 0; i < kMaxLogFileIndex; ++i) {
+    const base::FilePath new_filename(trace_file.value() + "." +
+                                      base::UintToString(i));
+    if (!base::PathExists(new_filename))
+      return new_filename;
+  }
+  return trace_file;
+}
+
+}  // namespace content