Add a map from layers to TabParents. (bug 1111873 part 1, r=smaug)

--HG--
extra : rebase_source : 68d9d718969ddffcfbbf44f03acbfe6c545b0965
Handle in-process APZ events correctly on Windows. (bug 1111873 part 2, r=kats)

--HG--
extra : rebase_source : 8ca5156392705e3a1806fc6a15154dbaa380f438