Don't add to LoadStart and LoadEnd histograms if load event hasn't fired yet.

chrome/renderer/page_load_histograms.cc

 // Copyright (c) 2010 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/renderer/page_load_histograms.h"
 
 #include "base/logging.h"
 #include "base/metrics/field_trial.h"
 #include "base/metrics/histogram.h"
 #include "base/time.h"
@@ skipping 88 matching lines @@
   missing_start_type |= commit.is_null() ? COMMIT_MISSING : 0;
   missing_start_type |= navigation_start.is_null() ? NAV_START_MISSING : 0;
   UMA_HISTOGRAM_ENUMERATION("PLT.MissingStart", missing_start_type,
                             MISSING_START_TYPE_MAX);
   if (missing_start_type)
     return;
 
   // Record the new PLT times prior to the faulty abandon check below.
   // TODO(tonyg): There are many new details we can record, add them after the
   // basic metrics are evaluated.
-  TimeDelta nav_start_to_load_start = load_event_start - navigation_start;
-  TimeDelta nav_start_to_load_end = load_event_end - navigation_start;
-  PLT_HISTOGRAM("PLT.NavStartToLoadStart", nav_start_to_load_start);
-  PLT_HISTOGRAM("PLT.NavStartToLoadEnd", nav_start_to_load_end);
+  // TODO(simonjam): There is no way to distinguish between abandonment and
+  // intentional Javascript navigation before the load event fires.
+  if (!load_event_start.is_null())
+    PLT_HISTOGRAM("PLT.NavStartToLoadStart",
+                  load_event_start - navigation_start);
+  if (!load_event_end.is_null())
+    PLT_HISTOGRAM("PLT.NavStartToLoadEnd", load_event_end - navigation_start);
 
   // We properly handle null values for the next 3 variables.
   Time request = navigation_state->request_time();
   Time first_paint = navigation_state->first_paint_time();
   Time first_paint_after_load = navigation_state->first_paint_after_load_time();
   Time finish_doc = navigation_state->finish_document_load_time();
   Time finish_all_loads = navigation_state->finish_load_time();
 
   // TODO(tonyg, jar): We suspect a bug in abandonment counting, this temporary
   // historgram should help us to troubleshoot.
@@ skipping 614 matching lines @@
 
   DCHECK(state);
   DCHECK(ds);
   GURL url(ds->request().url());
   Time start = state->start_load_time();
   Time finish = state->finish_load_time();
   // TODO(mbelshe): should we log more stats?
   VLOG(1) << "PLT: " << (finish - start).InMilliseconds() << "ms "
           << url.spec();
 }