Add compile time assert to help catch new fields

chrome/common/page_load_metrics/page_load_timing.cc

 // 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.
 
 #include "chrome/common/page_load_metrics/page_load_timing.h"
 
 namespace page_load_metrics {
 
 PageLoadTiming::PageLoadTiming() {}
 
 PageLoadTiming::PageLoadTiming(const PageLoadTiming& other) = default;
 
 PageLoadTiming::~PageLoadTiming() {}
 
+namespace {
+
+// *** IMPORTANT ***: If you change the number of TimeDeltas in PageLoadTiming,
+// also make sure to update the implementation of operator== and IsEmpty below
+// to reflect your changes (and update kNumTimeDeltas as well).
+const int kNumTimeDeltas = 13;
+
+const size_t kSizeOfTime = sizeof(base::Time);
+const size_t kSizeOfOptionalTimeDelta = sizeof(base::Optional<base::TimeDelta>);
+const size_t kSizeOfAllOptionalTimeDeltas =
+    kSizeOfOptionalTimeDelta * kNumTimeDeltas;
+static_assert(
+    sizeof(page_load_metrics::PageLoadTiming) ==
+        kSizeOfTime + kSizeOfAllOptionalTimeDeltas,
+    "Make sure to update IsEmpty and operator== to reflect your changes.");

[kinuko, 2016/08/01 15:38:00]

Could we always be sure that we have no padding between fields?

+
+}  // namespace
+
 bool PageLoadTiming::operator==(const PageLoadTiming& other) const {
   return navigation_start == other.navigation_start &&
          response_start == other.response_start &&
          dom_loading == other.dom_loading &&
          dom_content_loaded_event_start ==
              other.dom_content_loaded_event_start &&
          load_event_start == other.load_event_start &&
          first_layout == other.first_layout &&
          first_paint == other.first_paint &&
          first_text_paint == other.first_text_paint &&
@@ skipping 15 matching lines @@
          !parse_blocked_on_script_load_from_document_write_duration;
 }
 
 PageLoadMetadata::PageLoadMetadata() {}
 
 bool PageLoadMetadata::operator==(const PageLoadMetadata& other) const {
   return behavior_flags == other.behavior_flags;
 }
 
 }  // namespace page_load_metrics