More improvements to content script injection.

Don't continue injecting content scripts if the script injection process has been restarted (e.g. if scripts 1-3 of 5 have been injected, then a sub-frame completes loading so the whole process has to start again: don't inject script 4 which may depend on scripts 1-3).
But the background page survives across navigations? Doesn't this mean that event listeners are never cleared and you'll just steadily build up event listeners that don't do anything?