[page_load_metrics] Add support for chrome:// urls and add extensions observer

chrome/browser/page_load_metrics/observers/webui_page_load_metrics_observer.cc

+// Copyright 2016 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.
+
+#include "chrome/browser/page_load_metrics/observers/webui_page_load_metrics_observer.h"
+
+#include "chrome/browser/page_load_metrics/page_load_metrics_util.h"
+#include "chrome/common/url_constants.h"
+#include "content/public/browser/navigation_handle.h"
+
+namespace internal {
+
+const char kHistogramExtensionsParseDuration[] =
+    "PageLoad.Clients.WebUI.Extensions.ParseTiming."
+    "ParseDuration";
+
+}  // namespace internal
+WebUIPageLoadMetricsObserver::WebUIPageLoadMetricsObserver() {}
+
+page_load_metrics::TrackingPolicy WebUIPageLoadMetricsObserver::ShouldTrackLoad(
+    content::NavigationHandle* navigation_handle) {
+  const GURL& url = navigation_handle->GetURL();
+  if (url.SchemeIs("chrome")) {
+    if (url.path_piece().find(chrome::kChromeUIExtensionsHost) == 1) {

[Bryan McQuade, 2016/09/20 18:47:33]

can this become
if (url.host() == chrome::kChromeUIExtensionsHost)
?

[Charlie Harrison, 2016/09/20 22:46:23]

the chrome url gets rewritten to chrome://chrome/extensions. I saw this pattern
elsewhere for chrome urls...weird right?

+      tracked_page_ = EXTENSIONS;
+    }
+  }
+  return tracked_page_ == NONE ? page_load_metrics::IGNORE
+                               : page_load_metrics::TRACK;
+}
+
+void WebUIPageLoadMetricsObserver::OnParseStop(
+    const page_load_metrics::PageLoadTiming& timing,
+    const page_load_metrics::PageLoadExtraInfo& info) {
+  if (!WasStartedInForegroundOptionalEventInForeground(timing.parse_stop,
+                                                       info)) {
+    return;
+  }
+
+  switch (tracked_page_) {
+    case EXTENSIONS:
+      PAGE_LOAD_HISTOGRAM(
+          internal::kHistogramExtensionsParseDuration,

[Bryan McQuade, 2016/09/20 18:47:33]

is the concern for this page that it may spend a bunch of time loading script?
if so it may also be useful to track parse_blocked_on_script_load_duration as
well. if script exec time is a concern we can also track the exec variant.

[Charlie Harrison, 2016/09/20 22:46:23]

I mainly added this because it seemed useful to track the "many small resources"
case to optimize loading for (and other md-settings pages).

+devlin is this useful to you?

+          timing.parse_stop.value() - timing.parse_start.value());
+      break;
+    case NONE:
+      NOTREACHED();
+      break;
+  }
+}
+
+void WebUIPageLoadMetricsObserver::OnFirstContentfulPaint(
+    const page_load_metrics::PageLoadTiming& timing,
+    const page_load_metrics::PageLoadExtraInfo& info) {
+  if (!WasStartedInForegroundOptionalEventInForeground(
+          timing.first_contentful_paint, info)) {
+    return;
+  }
+
+  switch (tracked_page_) {
+    case EXTENSIONS:
+      PAGE_LOAD_HISTOGRAM(
+          "PageLoad.Clients.WebUI.Extensions.PaintTiming."
+          "NavigationToFirstContentfulPaint",
+          timing.first_contentful_paint.value());
+      break;
+    case NONE:
+      NOTREACHED();
+      break;
+  }
+}