Bug 1678652 - Disable failIfMajorPerformanceCaveat by default. r=lsalzman

The goal of the WebGL Working Group was for failIfMajorPerformanceCaveat
to allow websites to make better choices about which content to serve to
users, ideally via offering a choice to users.

I no longer think that this is useful here, or rather it's only ever
useful in the very narrow case: When a website does indeed have a
fallback ready, and the fallback does actually have better perf.

Today this is only true on some Windows systems that have cpu-emulated
webgl, but gpu-accelerated canvas2d, but this is not most systems.
Simultaneously, we've seen a disappointingly large set of websites break
for users that hit the failIfMajorPerformanceCaveat path.

Since the usefulness of failIfMajorPerformanceCaveat is very limited,
and many sites don't cooperate with us (as the user's User Agent), I
think it's best to disable failIfMajorPerformanceCaveat for the time
being.

Differential Revision: https://phabricator.services.mozilla.com/D100534

modules/libpref/init/StaticPrefList.yaml

@@ -10199,7 +10199,7 @@
 
 - name: webgl.disable-fail-if-major-performance-caveat
   type: RelaxedAtomicBool
-  value: false
+  value: true
   mirror: always
 
 - name: webgl.disable-DOM-blit-uploads