bug 1044125 - Only capture screenshot and show overlay when edge gesture...

...s are in use or task manager is used. r=alive
Haven't read through the patch, but maybe we could remove the screenshotOverlayState.
The existence of this state is because we have an asynchronous call inside this function, so we need to keep a state after we get the callback. Since you are removing the async call to getScreenshot I suspect we have any reason to keep it.