Bug 1539953 - Initialize mScrollId to the root scroll id. r=kvark

A spatial id of 0 refers to the root reference frame on the WR side, but
we shouldn't be using that on the Gecko side at all. Due to the
early-exit codepath in ClipManager we were actually sending some display
items with this spatial id over to WebRender. Although this doesn't
appear to cause any user-visible problems it seems wrong and can confuse
debugging other issues.

Differential Revision: https://phabricator.services.mozilla.com/D25296

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

gfx/layers/wr/ClipManager.cpp

@@ -370,7 +370,7 @@ ClipManager::ItemClips::ItemClips(const ActiveScrolledRoot* aASR,
                                   const DisplayItemClipChain* aChain,
                                   bool aSeparateLeaf)
     : mASR(aASR), mChain(aChain), mSeparateLeaf(aSeparateLeaf) {
-  mScrollId.id = 0;
+  mScrollId = wr::wr_root_scroll_node_id();
 }
 
 void ClipManager::ItemClips::UpdateSeparateLeaf(