Plumb experiment flags through page_load_metrics and add a new observer

chrome/browser/page_load_metrics/observers/document_write_page_load_metrics_observer.cc

Index: chrome/browser/page_load_metrics/observers/document_write_page_load_metrics_observer.cc
diff --git a/chrome/browser/page_load_metrics/observers/document_write_page_load_metrics_observer.cc b/chrome/browser/page_load_metrics/observers/document_write_page_load_metrics_observer.cc
new file mode 100644
index 0000000000000000000000000000000000000000..6347e424f3e5660f6ecd576cad934d86041baffa
--- /dev/null
+++ b/chrome/browser/page_load_metrics/observers/document_write_page_load_metrics_observer.cc
@@ -0,0 +1,78 @@
+// 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/document_write_page_load_metrics_observer.h"
+#include "components/page_load_metrics/browser/page_load_metrics_util.h"
+#include "third_party/WebKit/public/platform/WebLoadingBehaviorFlag.h"
+
+namespace internal {
+const char kHistogramDocWriteFirstContentfulPaint[] =

[Bryan McQuade, 2016/04/11 20:18:47]

it may end up being overkill, but i could imagine that this work could end up
shifting user behavior such that some pages that would've been backgrounded
before first paint will end up staying in foreground until first paint. to track
that, can we add background variants for the metrics you think it'd be useful
for?

[Charlie Harrison, 2016/04/12 15:18:50]

Done. I left out the parse complete metrics because I thought it would be
overkill.

+    "PageLoad.Clients.DocWrite.Evaluator.NavigationToFirstContentfulPaint";
+const char kHistogramDocWriteParseStartToFirstContentfulPaint[] =
+    "PageLoad.Clients.DocWrite.Evaluator.ParseStartToFirstContentfulPaint";
+const char kHistogramDocWriteParseDuration[] =
+    "PageLoad.Clients.DocWrite.Evaluator.ParseDuration";
+const char kHistogramDocWriteParseBlockedOnScript[] =
+    "PageLoad.Clients.DocWrite.Evaluator.ParseBlockedOnScriptLoad";
+const char kHistogramDocWriteParseBlockedOnScriptParseComplete[] =

[Bryan McQuade, 2016/04/11 20:18:47]

let's also add:
ParseBlockedOnScriptLoadFromDocumentWrite
ParseBlockedOnScriptLoadFromDocumentWrite.ParseComplete

(the data for these was added after you put this CL together)

[Charlie Harrison, 2016/04/12 15:18:50]

Done.

+    "PageLoad.Clients.DocWrite.Evaluator.ParseBlockedOnScriptLoad."
+    "ParseComplete";
+
+}  // namespace internal
+
+DocumentWritePageLoadMetricsObserver::DocumentWritePageLoadMetricsObserver() {}
+
+void DocumentWritePageLoadMetricsObserver::OnComplete(
+    const page_load_metrics::PageLoadTiming& timing,
+    const page_load_metrics::PageLoadExtraInfo& info) {
+  if (info.time_to_commit.is_zero() || timing.IsEmpty())
+    return;
+  if (info.metadata.behavior_flags &
+      blink::WebLoadingBehaviorFlag::WebLoadingBehaviorDocumentWriteEvaluator) {
+    LogDocumentWriteEvaluatorData(timing, info);
+  }
+}
+
+// TODO(csharrison,bmcquade): Log parse blocked on document write when that

[Bryan McQuade, 2016/04/11 20:18:47]

yep :) should be available now

+// metric becomes available.
+void DocumentWritePageLoadMetricsObserver::LogDocumentWriteEvaluatorData(
+    const page_load_metrics::PageLoadTiming& timing,
+    const page_load_metrics::PageLoadExtraInfo& info) {
+  bool foreground_paint = WasStartedInForegroundEventInForeground(
+      timing.first_contentful_paint, info);
+  if (foreground_paint) {
+    PAGE_LOAD_HISTOGRAM(internal::kHistogramDocWriteFirstContentfulPaint,
+                        timing.first_contentful_paint);
+  }
+
+  // Log parse based metrics.
+  if (!timing.parse_start.is_zero()) {
+    if (foreground_paint) {
+      PAGE_LOAD_HISTOGRAM(
+          internal::kHistogramDocWriteParseStartToFirstContentfulPaint,
+          timing.first_contentful_paint - timing.parse_start);
+    }
+
+    const bool incomplete_parse_in_foreground =
+        timing.parse_stop.is_zero() && info.started_in_foreground &&
+        info.first_background_time.is_zero();
+    if (incomplete_parse_in_foreground ||
+        WasStartedInForegroundEventInForeground(timing.parse_stop, info)) {
+      PAGE_LOAD_HISTOGRAM(internal::kHistogramDocWriteParseBlockedOnScript,
+                          timing.parse_blocked_on_script_load_duration);
+    }
+
+    // These metrics require a full parse.
+    if (!timing.parse_stop.is_zero()) {
+      base::TimeDelta parse_duration = timing.parse_stop - timing.parse_start;
+      if (WasStartedInForegroundEventInForeground(timing.parse_stop, info)) {
+        PAGE_LOAD_HISTOGRAM(internal::kHistogramDocWriteParseDuration,
+                            parse_duration);
+        PAGE_LOAD_HISTOGRAM(
+            internal::kHistogramDocWriteParseBlockedOnScriptParseComplete,
+            timing.parse_blocked_on_script_load_duration);
+      }
+    }
+  }
+}