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 b406334a5762d63f6899a269f0958afd61cff6d7..46628162ff23a4786ce9815a110397b9fd0bb5a3 100644
--- a/chrome/common/page_load_metrics/page_load_timing.cc
+++ b/chrome/common/page_load_metrics/page_load_timing.cc
@@ -12,6 +12,24 @@ 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 &&