Remove Cookie.SameSiteUnspecifiedEffective histogram

This histogram was about to expire, and is no longer interesting as
SameSite-by-default has launched which makes the samples almost entirely
Lax.

Bug: 1165039
Change-Id: I19bdb1ed423e198e7ffc22b2d24cfef81d6b9764
Fixed: 1165039
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2623001Reviewed-by: Maksim Orlovich <morlovich@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Lily Chen <chlily@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843006}

net/cookies/canonical_cookie.cc

@@ -713,13 +713,6 @@ CookieAccessResult CanonicalCookie::IncludeForRequestURL(
   CookieEffectiveSameSite effective_same_site =
       GetEffectiveSameSite(params.access_semantics);
   DCHECK(effective_same_site != CookieEffectiveSameSite::UNDEFINED);
-  // Log the effective SameSite mode that is applied to the cookie on this
-  // request, if its SameSite was not specified.
-  if (SameSite() == CookieSameSite::UNSPECIFIED) {
-    UMA_HISTOGRAM_ENUMERATION("Cookie.SameSiteUnspecifiedEffective",
-                              effective_same_site,
-                              CookieEffectiveSameSite::COUNT);
-  }
   UMA_HISTOGRAM_ENUMERATION(
       "Cookie.RequestSameSiteContext", cookie_inclusion_context,
       CookieOptions::SameSiteCookieContext::ContextType::COUNT);

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

@@ -540,6 +540,9 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 
 <histogram name="Cookie.SameSiteUnspecifiedEffective"
     enum="CookieEffectiveSameSite" expires_after="M90">
+  <obsolete>
+    Removed Jan 2021 for cleanup: crbug.com/1165039
+  </obsolete>
   <owner>chlily@chromium.org</owner>
   <owner>morlovich@chromium.org</owner>
   <summary>