Add metrics for cookie expiration dates with abbreviated 2-digit year

To assess compatibility impact of fixing behavior for year 69 (i.e.
change it from current behavior, 1969, to 2069, as described by the
spec).

Bug: 907610
Change-Id: Ia4639d34ca02144915aed47db4859aa545fae9c3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2605353Reviewed-by: Maksim Orlovich <morlovich@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Lily Chen <chlily@chromium.org>
Cr-Commit-Position: refs/heads/master@{#839803}

net/cookies/cookie_util.cc

@@ -11,6 +11,7 @@
 #include "base/callback.h"
 #include "base/check.h"
 #include "base/feature_list.h"
+#include "base/metrics/histogram_functions.h"
 #include "base/metrics/histogram_macros.h"
 #include "base/notreached.h"
 #include "base/stl_util.h"
@@ -313,6 +314,16 @@ base::Time ParseCookieExpirationTime(const std::string& time_string) {
     return base::Time();
   }
 
+  // Log metrics for use of abbreviated, 2-digit, year numbers to determine
+  // compat impact of fixing behavior for year 69.
+  // (crbug.com/907610) We currently treat 69 as 1969, whereas the spec says to
+  // treat it as 2069:
+  // https://tools.ietf.org/html/draft-ietf-httpbis-rfc6265bis-07#page-17
+  if (exploded.year >= 0 && exploded.year <= 99) {
+    base::UmaHistogramExactLinear("Cookie.AbbreviatedExpirationYear",
+                                  exploded.year, 100);
+  }
+
   // Normalize the year to expand abbreviated years to the full year.
   if (exploded.year >= 69 && exploded.year <= 99)
     exploded.year += 1900;

tools/metrics/histograms/histograms_xml/cookie/histograms.xml

@@ -21,6 +21,17 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 
 <histograms>
 
+<histogram name="Cookie.AbbreviatedExpirationYear" units="year"
+    expires_after="2021-06-30">
+  <owner>chlily@chromium.org</owner>
+  <owner>morlovich@chromium.org</owner>
+  <summary>
+    Records the abbreviated 2-digit year number used in a cookie's expiration
+    date. Emitted when parsing a cookie with an Expires attribute that specifies
+    an expiration year as a number between 0 and 99 inclusive.
+  </summary>
+</histogram>
+
 <histogram name="Cookie.BackingStoreUpdateResults" enum="BackingStoreResults"
     expires_after="M82">
   <owner>morlovich@chromium.org</owner>