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 {
+
+// The number of TimeDelta fields in PageLoadTiming.
+const int kNumTimeDeltas = 13;
+
+constexpr size_t ComputePageLoadTimingSize() {
+  // This assumes that the members of PageLoadTiming are stored without any
+  // padding. This is true for base::Time and base::Optional<base::TimeDelta>.
+  return sizeof(base::Time) /* sizeof navigation_start */ +
+         sizeof(base::Optional<base::TimeDelta>) * kNumTimeDeltas;
+}
+
+static_assert(
+    sizeof(PageLoadTiming) == ComputePageLoadTimingSize(),
+    "*** IMPORTANT ***: If you add new TimeDeltas to PageLoadTiming, update "
+    "operator==, IsEmpty, and kNumTimeDeltas to reflect your change.");
+
+}  // namespace
+
 bool PageLoadTiming::operator==(const PageLoadTiming& other) const {
   return navigation_start == other.navigation_start &&
          response_start == other.response_start &&
          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 &&
          first_image_paint == other.first_image_paint &&
@@ skipping 16 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