Bug 1409446 - Remove unused function. r=jrmuizel

We don't use the code to track the parents of each scroll id, so we can
dump it and just keep a set of scroll ids that we've defined to avoid
redefining them.

MozReview-Commit-ID: HY8y7xt9AJ6

--HG--
extra : rebase_source : 681154ae6494330f74c022243237b50d4921813b

gfx/webrender_bindings/WebRenderAPI.cpp

@@ -778,7 +778,7 @@ DisplayListBuilder::PopStickyFrame()
 bool
 DisplayListBuilder::IsScrollLayerDefined(layers::FrameMetrics::ViewID aScrollId) const
 {
-  return mScrollParents.find(aScrollId) != mScrollParents.end();
+  return mScrollIdsDefined.find(aScrollId) != mScrollIdsDefined.end();
 }
 
 void
@@ -794,9 +794,7 @@ DisplayListBuilder::DefineScrollLayer(const layers::FrameMetrics::ViewID& aScrol
       aAncestorClipId ? Stringify(aAncestorClipId.ref().id).c_str() : "(nil)",
       Stringify(aContentRect).c_str(), Stringify(aClipRect).c_str());
 
-  Maybe<layers::FrameMetrics::ViewID> parent =
-      mScrollIdStack.empty() ? Nothing() : Some(mScrollIdStack.back());
-  auto it = mScrollParents.insert({aScrollId, parent});
+  auto it = mScrollIdsDefined.insert(aScrollId);
   if (it.second) {
     // An insertion took place, which means we haven't defined aScrollId before.
     // So let's define it now.
@@ -806,10 +804,6 @@ DisplayListBuilder::DefineScrollLayer(const layers::FrameMetrics::ViewID& aScrol
     }
     wr_dp_define_scroll_layer(mWrState, aScrollId, aAncestorScrollId.ptrOr(nullptr),
         ancestorClipId, aContentRect, aClipRect);
-  } else {
-    // aScrollId was already a key in mScrollParents so check that the parent
-    // value is the same.
-    MOZ_ASSERT(it.first->second == parent);
   }
 }
 
@@ -1152,12 +1146,5 @@ DisplayListBuilder::TopmostScrollId()
   return mScrollIdStack.back();
 }
 
-Maybe<layers::FrameMetrics::ViewID>
-DisplayListBuilder::ParentScrollIdFor(layers::FrameMetrics::ViewID aScrollId)
-{
-  auto it = mScrollParents.find(aScrollId);
-  return (it == mScrollParents.end() ? Nothing() : it->second);
-}
-
 } // namespace wr
 } // namespace mozilla

gfx/webrender_bindings/WebRenderAPI.h

@@ -9,6 +9,7 @@
 
 #include <vector>
 #include <unordered_map>
+#include <unordered_set>
 
 #include "mozilla/AlreadyAddRefed.h"
 #include "mozilla/layers/SyncObject.h"
@@ -401,10 +402,6 @@ public:
   Maybe<wr::WrClipId> TopmostClipId();
   // Same as TopmostClipId() but for scroll layers.
   layers::FrameMetrics::ViewID TopmostScrollId();
-  // Returns the scroll id that was pushed just before the given scroll id. This
-  // function returns Nothing() if the given scrollid has not been encountered,
-  // or if it is the rootmost scroll id (and therefore has no ancestor).
-  Maybe<layers::FrameMetrics::ViewID> ParentScrollIdFor(layers::FrameMetrics::ViewID aScrollId);
 
   // Try to avoid using this when possible.
   wr::WrState* Raw() { return mWrState; }
@@ -422,10 +419,10 @@ protected:
   std::vector<wr::WrClipId> mClipIdStack;
   std::vector<layers::FrameMetrics::ViewID> mScrollIdStack;
 
-  // Track the parent scroll id of each scroll id that we encountered. A
-  // Nothing() value indicates a root scroll id. We also use this structure to
-  // ensure that we don't define a particular scroll layer multiple times.
-  std::unordered_map<layers::FrameMetrics::ViewID, Maybe<layers::FrameMetrics::ViewID>> mScrollParents;
+  // Track each scroll id that we encountered. We use this structure to
+  // ensure that we don't define a particular scroll layer multiple times,
+  // as that results in undefined behaviour in WR.
+  std::unordered_set<layers::FrameMetrics::ViewID> mScrollIdsDefined;
 
   // The number of extra clips that are in the stack.
   uint32_t mExtraClipCount;