Implement Brotli

[classilla]

This works in 46, so we need to undo the block and fix the outstanding bugs.

look at bugs 366559 and deps
1242904/1273547 (we might not need the upstream bump, but we should check)

[classilla]

Also, we should upgrade to the same build in ESR52 so we can get security fixes.

https://hg.mozilla.org/releases/mozilla-esr52/file/tip/modules/brotli/dec

[classilla]

Per https://bugzilla.mozilla.org/show_bug.cgi?id=1254411#c4 , cherry-picking the fix didn't work, so we should just grab the ESR version.

[classilla]

Looking at the source there appear to be some opportunities for AltiVec acceleration but we'll just stick with the C source for now.

[classilla]

Also pull netwerk/streamconv/converters/nsHTTPCompressConv.cpp up to ESR52. See and compare

https://hg.mozilla.org/releases/mozilla-esr52/log/tip/netwerk/streamconv/converters/nsHTTPCompressConv.cpp
https://hg.mozilla.org/releases/mozilla-esr45/log/tip/netwerk/streamconv/converters/nsHTTPCompressConv.cpp

[classilla]

This has been secretly landed. It appears to work, though there aren't many sites to test it against. The pref will not be activated until FPR1.