PSA: Change to running Gecko tests with WebRender enabled

[Kartikaya Gupta]

This is a heads up that once bug 1558598 (on autoland now) sticks it
will change how to run test suites with WebRender enabled to make
things simpler and more consistent.

Previously, if you ran tests with `./mach <whatever>` WebRender may or
may not have been enabled depending on the hardware being used. To
force it on or off you'd have to either set the preference (e.g.
gfx.webrender.all=true) or set environment variables (e.g.
MOZ_WEBRENDER=1).

With bug 1558598, this is no longer the case. Now, WebRender is
disabled unless the test is explicitly run with `--enable-webrender`,
regardless of underlying hardware. So e.g. if you run `MOZ_WEBRENDER=1
./mach reftest foo` this will run with WebRender disabled, because you
didn't pass `--enable-webrender` to `./mach reftest` and it will
override your `MOZ_WEBRENDER=1` and disable WebRender anyway. The same
goes for the pref.

All the test harnesses I could find (junit, gtest, mochitest, reftest,
WPT, raptor, talos, cppunit, xpcshell, marionette, marionette
derivatives, and jit) now support the `--enable-webrender` flag and
expose them via their corresponding mach commands. Mozharness scripts
used to run these tests have also been updated so tests running in
taskcluster automation are similarly consistent. This also works when
running tests on both desktop and Android (WR is supported only on
GeckoView-based things, not Fennec).

There used to be a `--disable-webrender` flag on some harnesses, which
has been removed as redundant; simply omitting `--enable-webrender`
will forcibly disable WebRender. Also, mozinfo (the thing used to
evaluate the webrender condition in mochitest.ini/xpcshell.ini
annotations and WPT meta files) is also now keying off the presence of
the `--enable-webrender` flag and so should behave consistently with
local test runs. Previously it was possible to be running tests with
WR enabled in a way that mozinfo would think it was disabled, which
should no longer be the case.

Please let me know if you run into any issues or have questions.

Cheers,
kats