Add UMA metric: Startup.BrowserMainToRendererMain.

components/startup_metric_utils/browser/startup_metric_message_filter.h

Index: components/startup_metric_utils/browser/startup_metric_message_filter.h
diff --git a/components/startup_metric_utils/browser/startup_metric_message_filter.h b/components/startup_metric_utils/browser/startup_metric_message_filter.h
new file mode 100644
index 0000000000000000000000000000000000000000..282ffa5c03deacb66d987be405401ac22d55648a
--- /dev/null
+++ b/components/startup_metric_utils/browser/startup_metric_message_filter.h
@@ -0,0 +1,29 @@
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef COMPONENTS_STARTUP_METRIC_UTILS_BROWSER_STARTUP_METRIC_MESSAGE_FILTER_H_
+#define COMPONENTS_STARTUP_METRIC_UTILS_BROWSER_STARTUP_METRIC_MESSAGE_FILTER_H_
+
+#include <stdint.h>
+
+#include "base/macros.h"
+#include "content/public/browser/browser_message_filter.h"
+
+namespace startup_metric_utils {
+
+class StartupMetricMessageFilter : public content::BrowserMessageFilter {
+ public:
+  StartupMetricMessageFilter();

[gab, 2015/11/06 18:05:17]

I think the style now allows to use 
StartupMetricMessageFilter() = default;
inline here (same for destructor).

(the rule being that inline constructor/destructor are allowed IFF your class
has no non-POD members)

[fdoray, 2015/11/09 15:53:56]

Done, for the destructor. The constructor cannot be default (has to pass
StartupMetricMsgStart to the base class constructor).

Note that the base class has non-POD members. Is it still OK to use =default in
that case?

The style guide
https://google.github.io/styleguide/cppguide.html#Inline_Functions says "Beware
of destructors, which are often longer than they appear because of implicit
member- and base-destructor calls!"

[gab, 2015/11/09 16:01:28]

Yes, essentially inlining this class' destructor will end up inlining the base
class' destructor in the caller (which itself will be a jump to the impl of that
destructor if it turns out to not be inlinable).

+  bool OnMessageReceived(const IPC::Message& message) override;
+
+ private:
+  ~StartupMetricMessageFilter() override;
+  void OnRecordRendererMainEntryTime(int64_t renderer_main_entry_time);
+
+  DISALLOW_COPY_AND_ASSIGN(StartupMetricMessageFilter);
+};
+
+}  // namespace startup_metric_utils
+
+#endif  // COMPONENTS_STARTUP_METRIC_UTILS_BROWSER_STARTUP_METRIC_MESSAGE_FILTER_H_