Bug 1503616 - Set a animatable transform property on the root content WR stacking context. r=mstange

This sets a placeholder transform on the root stacking context for the
content display list, and allows us to modify that transform directly in
the compositor. This is similar to what
nsDisplayOwnLayer::CreateWebRenderCommands does for scroll thumb layers,
which are similarly manipulated by APZ for async scrolling.

Differential Revision: https://phabricator.services.mozilla.com/D11180

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

gfx/layers/wr/WebRenderCommandBuilder.cpp

@@ -1321,7 +1321,14 @@ WebRenderCommandBuilder::BuildWebRenderCommands(wr::DisplayListBuilder& aBuilder
   mClipManager.BeginBuild(mManager, aBuilder);
 
   {
-    StackingContextHelper pageRootSc(sc, nullptr, aBuilder, aFilters);
+    if (!mZoomProp && gfxPrefs::APZAllowZooming() && XRE_IsContentProcess()) {
+      mZoomProp.emplace();
+      mZoomProp->effect_type = wr::WrAnimationType::Transform;
+      mZoomProp->id = AnimationHelper::GetNextCompositorAnimationsId();
+    }
+
+    StackingContextHelper pageRootSc(sc, nullptr, aBuilder, aFilters,
+        LayoutDeviceRect(), nullptr, mZoomProp.ptrOr(nullptr));
     if (ShouldDumpDisplayList(aDisplayListBuilder)) {
       mBuilderDumpIndex = aBuilder.Dump(mDumpIndent + 1, Some(mBuilderDumpIndex), Nothing());
     }

gfx/layers/wr/WebRenderCommandBuilder.h

@@ -193,6 +193,11 @@ private:
 
   wr::usize mBuilderDumpIndex;
   wr::usize mDumpIndent;
+
+  // When zooming is enabled, this stores the animation property that we use
+  // to manipulate the zoom from APZ.
+  Maybe<wr::WrAnimationProperty> mZoomProp;
+
 public:
   // Whether consecutive inactive display items should be grouped into one
   // blob image.