Bug 1441308 - Add the render root splitting pref r=sotaro

Differential Revision: https://phabricator.services.mozilla.com/D12844

--HG--
extra : moz-landing-system : lando

gfx/thebes/gfxPrefs.h

@@ -550,6 +550,7 @@ class gfxPrefs final {
   DECL_GFX_PREF(Live, "gfx.webrender.late-scenebuild-threshold", WebRenderLateSceneBuildThreshold, int32_t, 4);
   DECL_GFX_PREF(Live, "gfx.webrender.max-filter-ops-per-chain", WebRenderMaxFilterOpsPerChain, uint32_t, 64);
   DECL_GFX_PREF(Live, "gfx.webrender.picture-caching",         WebRenderPictureCaching, bool, false);
+  DECL_GFX_PREF(Once, "gfx.webrender.split-render-roots",      WebRenderSplitRenderRoots, bool, false);
 
   // Use vsync events generated by hardware
   DECL_GFX_PREF(Once, "gfx.work-around-driver-bugs",           WorkAroundDriverBugs, bool, true);