Bug 1622360 - Remove mRootStackingContexts. r=jrmuizel

This isn't used any more; the only getter function is not called from
anywhere.

Depends on D70602

Differential Revision: https://phabricator.services.mozilla.com/D70603

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

gfx/layers/wr/WebRenderCommandBuilder.cpp

@@ -1477,7 +1477,6 @@ void WebRenderCommandBuilder::DoGroupingForDisplayList(
 WebRenderCommandBuilder::WebRenderCommandBuilder(
     WebRenderLayerManager* aManager)
     : mManager(aManager),
-      mRootStackingContexts(nullptr),
       mLastAsr(nullptr),
       mBuilderDumpIndex(0),
       mDumpIndent(0),
@@ -1553,10 +1552,6 @@ void WebRenderCommandBuilder::BuildWebRenderCommands(
       mBuilderDumpIndex =
           aBuilder.Dump(mDumpIndent + 1, Some(mBuilderDumpIndex), Nothing());
     }
-    MOZ_ASSERT(mRootStackingContexts == nullptr);
-    AutoRestore<wr::RenderRootArray<Maybe<StackingContextHelper>>*> rootScs(
-        mRootStackingContexts);
-    mRootStackingContexts = &pageRootScs;
     CreateWebRenderCommandsFromDisplayList(
         aDisplayList, nullptr, aDisplayListBuilder,
         *pageRootScs[wr::RenderRoot::Default], aBuilder, aResourceUpdates);

gfx/layers/wr/WebRenderCommandBuilder.h

@@ -113,11 +113,6 @@ class WebRenderCommandBuilder final {
 
   bool GetContainsSVGGroup() { return mContainsSVGGroup; }
 
-  const StackingContextHelper& GetRootStackingContextHelper(
-      wr::RenderRoot aRenderRoot) const {
-    return *(*mRootStackingContexts)[aRenderRoot];
-  }
-
   // Those are data that we kept between transactions. We used to cache some
   // data in the layer. But in layers free mode, we don't have layer which
   // means we need some other place to cached the data between transaction.
@@ -182,7 +177,6 @@ class WebRenderCommandBuilder final {
                                const StackingContextHelper& aSc,
                                nsDisplayListBuilder* aDisplayListBuilder);
 
-  wr::RenderRootArray<Maybe<StackingContextHelper>>* mRootStackingContexts;
   ClipManager mClipManager;
 
   // We use this as a temporary data structure while building the mScrollData