Adds googLeakyBucket flag to screen sharing mandatory constraints.

libs/strophe/strophe.jingle.adapter.js

         constraints.video = {
             mandatory: {
                 chromeMediaSource: "screen",
+                googLeakyBucket: true,
                 maxWidth: window.screen.width,
                 maxHeight: window.screen.height,
                 maxFrameRate: 3
             mandatory: {
                 chromeMediaSource: "desktop",
                 chromeMediaSourceId: desktopStream,
+                googLeakyBucket: true,
                 maxWidth: window.screen.width,
                 maxHeight: window.screen.height,
                 maxFrameRate: 3