Bug 1657822. Enable new desktop zooming scrollbar code. r=kats

The mathml reftest pref needs to be added at the same time because the test needs that pref with the new scrollbar code to pass, but it fails with that pref with the old scrollbar code. Since that test uses the minimum scale, which is only valid with overlay scrollbars, it is correct to do so.

Differential Revision: https://phabricator.services.mozilla.com/D91243

gfx/thebes/gfxPlatform.cpp

@@ -3256,7 +3256,6 @@ uint32_t gfxPlatform::TargetFrameRate() {
 
 /* static */
 bool gfxPlatform::UseDesktopZoomingScrollbars() {
-  // bug 1657822 to enable this by default
   return StaticPrefs::apz_allow_zooming() &&
          !StaticPrefs::apz_force_disable_desktop_zooming_scrollbars();
 }

layout/reftests/mathml/reftest.list

@@ -365,7 +365,7 @@ pref(mathml.deprecated_alignment_attributes.disabled,false) fuzzy-if(geckoview&&
 pref(mathml.deprecated_alignment_attributes.disabled,false) fuzzy-if(geckoview&&webrender,0-239,0-271) == mfrac-D-4.html mfrac-D-4-ref.html
 == mfrac-E-1.html mfrac-E-1-ref.html
 == shadow-dom-1.html shadow-dom-1-ref.html
-pref(dom.meta-viewport.enabled,true) pref(font.size.inflation.emPerLine,25) fuzzy-if(webrender&&!gtkWidget,0-255,0-324) fuzzy-if(webrender&&gtkWidget,0-255,0-66) == font-inflation-1.html font-inflation-1-ref.html  # gtkWidget due to Bug 1607294
+pref(ui.useOverlayScrollbars,1) pref(dom.meta-viewport.enabled,true) pref(font.size.inflation.emPerLine,25) fuzzy-if(webrender&&!gtkWidget,0-255,0-324) fuzzy-if(webrender&&gtkWidget,0-255,0-66) == font-inflation-1.html font-inflation-1-ref.html  # gtkWidget due to Bug 1607294
 test-pref(font.minimum-size.x-math,40) == default-font.html default-font-ref.html
 != radicalbar-1.html about:blank
 != radicalbar-1a.html about:blank

modules/libpref/init/StaticPrefList.yaml

@@ -459,7 +459,7 @@
 # new scrollbar code for desktop zooming
 - name: apz.force_disable_desktop_zooming_scrollbars
   type: RelaxedAtomicBool
-  value: true
+  value: @IS_ANDROID@
   mirror: always
 
 #ifdef MOZ_WIDGET_GTK