This patch contains two parts below:
1. mNeckoTarget
Since almost all child channels in necko need mNeckoTarget to dispatch runnables to main thread, it seems worth to have a holder class of mNeckoTarget and ask child channels to inherit it.
2. ChannelEvent
A lot of classes in FTPChannelChild and HttpChannelChild inheriting ChannelEvent and override GetEventTarget. It should be worth to extract the same code and put it in the same space.
