[e10s] Private protocol doesn't work in multi-process mode

[Infocatcher]

(related to #162) https://bugzilla.mozilla.org/show_bug.cgi?id=1114946#c7 Fix/workaround: https://bugzilla.mozilla.org/show_bug.cgi?id=940206#c22 https://bugzilla.mozilla.org/show_bug.cgi?id=1049284 - in Custom Buttons, now wontfix'ed https://bugzilla.mozilla.org/show_bug.cgi?id=1095484 - in ChatZilla

[Infocatcher]

What we have for now:

• one parent process (with already registered nsIProtocolHandler implementation)
• one child process (for now?), will be unloaded (and nsIProtocolHandler implementation will be destroyed) after closing of last remote tab
• so, we should load our "registrar" into each tab and each new opened tab (using global frame script)
• our frame script will be cached (yay!): https://bugzilla.mozilla.org/show_bug.cgi?id=1051238
• will use "content.js?" + Date.now() or similar
• also we should have jsm for protocol things to use only one code in each tab/frame
• and frame script will be executed in non-remote tabs too (and we already have "global" nsIProtocolHandler implementation)
• may be something else