gnome-session spamming syslog

[Colin Law]

Since switch to systemd on vivid I am seeing gnome-session spamming
syslog every 30 seconds or so with

Mar  9 15:57:34 tigger gnome-session[1849]: WARNING: content window
passed to PrivateBrowsingUtils.isWindowPrivate. Use
isContentWindowPrivate instead (but only for frame scripts).
Mar  9 15:57:34 tigger gnome-session[1849]:
pbu_isWindowPrivate at resource://gre/modules/PrivateBrowsingUtils.jsm:25:14
Mar  9 15:57:34 tigger gnome-session[1849]:
NM__onRequestHeader at resource://gre/modules/commonjs/toolkit/loader.js
-> resource://gre/modules/devtools/toolkit/webconsole/network-monitor.js:659:34
Mar  9 15:57:34 tigger gnome-session[1849]:
NM_observeActivity at resource://gre/modules/commonjs/toolkit/loader.js
-> resource://gre/modules/devtools/toolkit/webconsole/network-monitor.js:532:7
Mar  9 15:57:34 tigger gnome-session[1849]:
makeInfallible/<@resource://gre/modules/commonjs/toolkit/loader.js ->
resource://gre/modules/devtools/DevToolsUtils.js:83:14

Is this a bug or is it just that we have some verbose output on
somewhere which will get turned off in the fullness of time?

Colin