e10s frame script utils usage

test-e10s-mm.js

const { classes: Cc, interfaces: Ci, utils: Cu, results: Cr } = Components;
const FRAME_SCRIPT_UTILS_URL = "chrome://browser/content/devtools/frame-script-utils.js"

function navigateInHistory(aTarget, aDirection, aWaitForTargetEvent = "navigate") {
  // if e10s, use the message manager
  if (Cu.isCrossProcessWrapper(content)) {
    let mm = gBrowser.selectedBrowser.messageManager;
    mm.loadFrameScript(FRAME_SCRIPT_UTILS_URL, false);
    mm.sendAsyncMessage("devtools:test:history", { direction: aDirection });
  }
  // Otherwise just access content directly
  else {
    executeSoon(() => content.history[aDirection]());
  }
  return once(aTarget, aWaitForTargetEvent);
}

actually, according to billm:

@billm> zombie_: it should be fine to do loadFrameScript and then sendAsyncMessage. the message won't be processed until the frame script has run.
00:51 @billm zombie_: so as long as the frame script immediately adds a listener for the message, it will work