Bug 919325 - Minimize the #includes in content/ IDL files; r=bzbarsky

--HG--
extra : rebase_source : 923a665eadc62046c9d5398a25187a3331fd8d25
Bug 919326 - Minimize the #includes in layout/ IDL files; r=roc

--HG--
extra : rebase_source : ec8cdff9466ff75463841818537ecead2cb57e62
Bug 919330 - Minimize the #includes in docshell/ IDL files; r=bzbarsky

--HG--
extra : rebase_source : ddae0c7f915c086fcd98bda168ab127764d4f7fc