Migration to FPR7

[OlgaTPark]

Additional patches from Firefox ESR52 for FPR7 on Intel

This bug documents things took from https://github.com/classilla/tenfourfox/issues/491.

Not taking:

• [ ] M1451376 → We don't have e10s;

• [ ] M1444668 → No wasm;

• [ ] M1452619 → We don't support Windows™;

• [ ] M1453339 → I cannot trigger the testcase, also in the fully-patched FPR7;

  Wontfix:

• [ ] M1446365 → (As explained in OlgaTPark/tenfourfox#12) The libtremor part of this patch isn't in TenFourFox (the remainder is in FPR7b1). libtremor is a integer-only replacement for libvorbis which lacks an encoder (it's unused in TFX and only used on ARM or Android — the two are mutually exclusive). Wontfixing it (so don't force MOZ_TREMOR to 1…);

  Taking:

• [x] M1409440 → In code and Skia is re-enabled on TenSixFox;

• [x] M1441941 → Skia is re-enabled in TenSixFox (just replace std::numeric_limits for pre-C++11 nevermind, this is available in C++98 but skia is missing a #include <limits> (without .h));

• [x] M1454692 → Skia is re-enabled in TenSixFox;

• [x] M1440717 → WebGL is re-enabled in TenFiveFox and TenSixFox;

• [x] M1426129 → Don't know if affected BUT WebRTC is re-enabled in this fork (Note: since we don't have M1308114, the method CamerasChild::EnsureInitialized is not in code);

• [x] M1361699 → Not really a security issue but still interesting since users are probably using a good old HDD (just mark nsCheckSummedOutputStream class as final for performance);

My Shopping list:

• [x] Import classilla/tenfourfox#619 (https://github.com/classilla/tenfourfox/commit/c2d4942155b4448e34098666f97add33ff280a6d) in my release in order to allow everyone to use my (too) old branch.

Others:

• M1449898 is already fixed by M1452075 in https://github.com/classilla/tenfourfox/commit/7b3c79d41bc7324738e652256d86dc77249c3237;