Miscellaneous performance improvements

[classilla]

These should stand alone:
https://bugzilla.mozilla.org/show_bug.cgi?id=1331342
https://bugzilla.mozilla.org/show_bug.cgi?id=1350770
https://hg.mozilla.org/mozilla-central/rev/2c670b3e33db (note change in filename)

Small JS string assistance. Apply in order:
https://hg.mozilla.org/mozilla-central/rev/1f5902db90a7
https://hg.mozilla.org/mozilla-central/rev/4cda93365031

Fast path for empty frames. In order:
https://hg.mozilla.org/mozilla-central/rev/c30756846670
https://hg.mozilla.org/mozilla-central/rev/63a14b2f3c74

[classilla]

Easy win for spellcheck (we use case nsISelectionController::SELECTION_SPELLCHECK but it's the same code in the same location, approximately).
https://hg.mozilla.org/mozilla-central/rev/abfece312d74

[classilla]

Kind of a duh change.
https://hg.mozilla.org/mozilla-central/rev/1fb8edcd2a7c

[classilla]

This is a consideration after the string changes above land, but there might be some risk in it given that it changes caching performance.
https://bugzilla.mozilla.org/show_bug.cgi?id=1353758

[classilla]

We do have the block-scoped ReentrantMonitor XPCOM object, so this should be nice to have also.
https://bugzilla.mozilla.org/show_bug.cgi?id=1354389

[classilla]

This one is really easy.
https://hg.mozilla.org/mozilla-central/rev/609c12db5796

[classilla]

Another duh change.
https://hg.mozilla.org/mozilla-central/rev/630ecf1e615d

[classilla]

This needs a couple backbugs (see the event.target code), but should work (we do have event.originalTarget and ._scrollbox).

https://hg.mozilla.org/mozilla-central/rev/c0e6cf04adb9

[classilla]

We could consider this after the 'duh' change directly above but the comments imply this isn't really all that worth it.
https://hg.mozilla.org/mozilla-central/rev/ee9bd670aadf

[classilla]

Another easy change.
https://bugzilla.mozilla.org/show_bug.cgi?id=1349742

[classilla]

There are a couple of leftover telemetry things this removes which we should also be able to do safely. (Also remove the bind(this))
https://hg.mozilla.org/mozilla-central/rev/1685702d2d6a

[classilla]

Another nice win.
https://hg.mozilla.org/mozilla-central/rev/7640ebe4a0e2

[classilla]

Another gimme. I love Photon.
https://hg.mozilla.org/mozilla-central/rev/c5bdfc3ba44d

[classilla]

We don't need the second patch which is just a rename. The performance implications are unclear since we are not e10s by design, but it seems like it would certainly help correctness as well.
https://hg.mozilla.org/mozilla-central/rev/c5bdfc3ba44d
However, verify this does not cause https://bugzilla.mozilla.org/show_bug.cgi?id=1357841

(Note to self: subscribe to these bugs, too, so we get regression alerts)

[classilla]

Improve pref management for IME.
https://hg.mozilla.org/mozilla-central/rev/f0127a117bcb

[classilla]

Eliminate a layout flush.
https://hg.mozilla.org/mozilla-central/rev/1be01419533e