Accessing interest groups from the same site

To fix WICG#162.

FLEDGE.md

@@ -275,7 +275,7 @@ Once the trusted bidding signals are fetched, each interest group's bidding func
 
 
 ```
-generateBid(interestGroup, auctionSignals, perBuyerSignals, trustedBiddingSignals, browserSignals) {
+generateBid(interestGroup, auctionSignals, perBuyerSignals, trustedBiddingSignals, browserSignals, sameSiteInterestGroups) {
   ...
   return {'ad': adObject, 'bid': bidValue, 'render': renderUrl, 'adComponents': [adComponent1, adComponent2, ...]};
 }
@@ -299,6 +299,7 @@ The arguments to `generateBid()` are:
       'prevWins': [[time1,ad1],[time2,ad2],...],
     }
     ```
+* sameSiteInterestGroups: if the user is added to `interestGroup` on a single site, this argement contains a list of all other interest group objects added by that site; if the user is added to `interestGroup` on mutliple sites, this argument will be empty.
 
 The output of `generateBid()` contains four fields: