Pull user device list on join

When a new user joins a room, make sure we download their device list if we
don't already have it.

This should fix at least one cause of
https://github.com/vector-im/vector-web/issues/2249.

lib/crypto/algorithms/megolm.js

@@ -169,7 +169,11 @@ MegolmEncryption.prototype._prepareNewSession = function(room) {
  * @private
  *
  * @param {string} session_id
+ *
  * @param {Object<string, Object<string, boolean>|boolean>} shareMap
+ *    Map from userid to true (meaning this is a new user in the room, so all
+ *    of his devices need the keys), or a map from deviceid to true (meaning
+ *    this user has one or more new devices, which need the keys).
  *
  * @return {module:client.Promise} Promise which resolves once the key sharing
  *     message has been sent.
@@ -189,6 +193,9 @@ MegolmEncryption.prototype._shareKeyWithDevices = function(session_id, shareMap)
         }
     };
 
+    // we downloaded the user's device list when they joined the room, or when
+    // the new device announced itself, so there is no need to do so now.
+
     return self._crypto.ensureOlmSessionsForUsers(
         utils.keys(shareMap)
     ).then(function(devicemap) {
@@ -292,8 +299,7 @@ MegolmEncryption.prototype.onRoomMembership = function(event, member, oldMembers
     var newMembership = member.membership;
 
     if (newMembership === 'join') {
-        // new member in the room.
-        this._devicesPendingKeyShare[member.userId] = true;
+        this._onNewRoomMember(member.userId);
         return;
     }
 
@@ -318,6 +324,23 @@ MegolmEncryption.prototype.onRoomMembership = function(event, member, oldMembers
     }
 };
 
+/**
+ * handle a new user joining a room
+ *
+ * @param {string} userId   new member
+ */
+MegolmEncryption.prototype._onNewRoomMember = function(userId) {
+    // make sure we have a list of this user's devices. We are happy to use a
+    // cached version here: we assume that if we already have a list of the
+    // user's devices, then we already share an e2e room with them, which means
+    // that they will have announced any new devices via an m.new_device.
+    this._crypto.downloadKeys([userId], false).done();
+
+    // also flag this user up for needing a keyshare.
+    this._devicesPendingKeyShare[userId] = true;
+};
+
+
 /**
  * @inheritdoc
  *