Bug 1564929 - Stop needless round-trip to fake app units. r=Gankro

Also move the impl to the .cpp file to avoid expensive rebuilds when it
is modified.

Differential Revision: https://phabricator.services.mozilla.com/D37589

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

layout/painting/nsDisplayList.cpp

@@ -1593,6 +1593,15 @@ uint32_t nsDisplayListBuilder::GetImageDecodeFlags() const {
   return flags;
 }
 
+void nsDisplayListBuilder::ComputeDefaultRenderRootRect(LayoutDeviceIntSize aClientSize) {
+  LayoutDeviceIntRegion cutout;
+  LayoutDeviceIntRect clientRect(LayoutDeviceIntPoint(), aClientSize);
+  cutout.OrWith(clientRect);
+  cutout.SubOut(RoundedToInt(mRenderRootRects[mozilla::wr::RenderRoot::Content]));
+
+  mRenderRootRects[mozilla::wr::RenderRoot::Default] = LayoutDeviceRect(cutout.GetBounds());
+}
+
 void nsDisplayListBuilder::SubtractFromVisibleRegion(nsRegion* aVisibleRegion,
                                                      const nsRegion& aRegion) {
   if (aRegion.IsEmpty()) {

layout/painting/nsDisplayList.h

@@ -936,17 +936,7 @@ class nsDisplayListBuilder {
     return mNeedsDisplayListBuild[aRenderRoot];
   }
 
-  void ComputeDefaultRenderRootRect(LayoutDeviceIntSize aClientSize) {
-    nsRegion cutout;
-    nsRect clientRect(nsPoint(),
-                      mozilla::LayoutDevicePixel::ToAppUnits(aClientSize, 1));
-    cutout.OrWith(clientRect);
-    cutout.SubWith(mozilla::LayoutDevicePixel::ToAppUnits(
-        mRenderRootRects[mozilla::wr::RenderRoot::Content], 1));
-
-    mRenderRootRects[mozilla::wr::RenderRoot::Default] =
-        mozilla::LayoutDevicePixel::FromAppUnits(cutout.GetBounds(), 1);
-  }
+  void ComputeDefaultRenderRootRect(LayoutDeviceIntSize aClientSize);
 
   LayoutDeviceRect GetRenderRootRect(mozilla::wr::RenderRoot aRenderRoot) {
     return mRenderRootRects[aRenderRoot];