[Spec] Add recency for generateBid(). (#711)

spec.bs

@@ -284,6 +284,7 @@ This is detectable because it can change the set of fields that are read from th
     the currently stored one from the browser.
   1. Set |interestGroup|'s [=interest group/joining origin=] to [=this=]'s
     [=relevant settings object=]'s [=environment/top-level origin=].
+  1. Set |interestGroup|'s [=interest group/join time=] to the [=current wall time=].
   1. If the most recent entry in |interestGroup|'s [=interest group/join counts=] corresponds to
     the current day in UTC, increment its count. If not, [=list/insert=] a new [=tuple=]
     the time set to the current UTC day and a count of 1.
@@ -1070,6 +1071,8 @@ an [=interest group=] |ig|, and a [=moment=] |auctionStartTime|:
     with |ig|.
   1. Set |browserSignals|["{{BiddingBrowserSignals/joinCount}}"] to the sum of |ig|'s
      [=interest group/join counts=] for all days within the last 30 days.
+  1. Set |browserSignals|["{{BiddingBrowserSignals/recency}}"] to the [=current wall time=]
+    minus |ig|'s [=interest group/join time=], in milliseconds.
   1. [=map/Set=] |browserSignals|["{{BiddingBrowserSignals/bidCount}}"] to the sum of |ig|'s
      [=interest group/bid counts=] for all days within the last 30 days.
   1. Let |prevWins| be a new <code>[=sequence=]<{{PreviousWin}}></code>.
@@ -2802,6 +2805,7 @@ dictionary BiddingBrowserSignals {
   required USVString seller;
   required long joinCount;
   required long bidCount;
+  required long recency;
 
   USVString topLevelSeller;
   sequence<PreviousWin> prevWinsMs;
@@ -3010,6 +3014,9 @@ An interest group is a [=struct=] with the following [=struct/items=]:
   is calculated based on UTC time. The join count is a count of the number of
   times {{Navigator/joinAdInterestGroup()}} was called for this interest group on the
   corresponding day.
+: <dfn>join time</dfn>
+:: A [=moment=] at which the browser joined this interest group, updated upon each join and
+  re-join.
 : <dfn>bid counts</dfn>
 :: A [=list=] containing [=tuples=] of the day and per day bid count. The day
   is calculated based on UTC time. The bid count is a count of the number of