Add compile time assert to help catch new fields

chrome/common/page_load_metrics/page_load_timing.cc

Index: chrome/common/page_load_metrics/page_load_timing.cc
diff --git a/chrome/common/page_load_metrics/page_load_timing.cc b/chrome/common/page_load_metrics/page_load_timing.cc
index d96844881c72490c9aa961af1ed3f5712b36fe83..5244d1082348d54203c186a718979730032cb4c9 100644
--- a/chrome/common/page_load_metrics/page_load_timing.cc
+++ b/chrome/common/page_load_metrics/page_load_timing.cc
@@ -12,6 +12,25 @@ 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 &&