Firefox pdf viewer support for e10s (Bug 942707).

See https://bugzilla.mozilla.org/show_bug.cgi?id=942707
During disable:
WARN	Exception running bootstrap method startup on uriloader@pdf.js: [Exception... "Component returned failure code: 0xc1f30100 (NS_ERROR_FACTORY_EXISTS) [nsIComponentRegistrar.registerFactory]"  nsresult: "0xc1f30100 (NS_ERROR_FACTORY_EXISTS)"  location: "JS frame :: resource://gre/modules/addons/XPIProvider.jsm -> jar:file:///Users/bdahl/Library/Application%20Support/Firefox/Profiles/dx3hp1j0.e10s/extensions/uriloader@pdf.js.xpi!/bootstrap.js :: register :: line 99"  data: no] Stack trace: register()@resource://gre/modules/addons/XPIProvider.jsm -> jar:file:///Users/bdahl/Library/Application%20Support/Firefox/Profiles/dx3hp1j0.e10s/extensions/uriloader@pdf.js.xpi!/bootstrap.js:99 < startup()@resource://gre/modules/addons/XPIProvider.jsm -> jar:file:///Users/bdahl/Library/Application%20Support/Firefox/Profiles/dx3hp1j0.e10s/extensions/uriloader@pdf.js.xpi!/bootstrap.js:152 < XPI_callBootstrapMethod()@resource://gre/modules/addons/XPIProvider.jsm:4304 < XPI_updateAddonDisabledState()@resource://gre/modules/addons/XPIProvider.jsm:4427 < AddonWrapper_userDisabledSetter()@resource://gre/modules/addons/XPIProvider.jsm:6741 < set_userDisabled()@extensions.xml:1084 < oncommand()@about:addons:1 < <file:unknown>