Bug 1559546 - Allow to construct StyleAtom objects from C++. r=heycam

I'm going to need it in an upcoming patch.

Differential Revision: https://phabricator.services.mozilla.com/D35118

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

layout/style/ServoStyleConstsInlines.h

@@ -242,6 +242,19 @@ inline StyleAtom::~StyleAtom() {
   }
 }
 
+inline StyleAtom::StyleAtom(already_AddRefed<nsAtom> aAtom) {
+  nsAtom* atom = aAtom.take();
+  if (atom->IsStatic()) {
+    ptrdiff_t offset = reinterpret_cast<const uint8_t*>(atom->AsStatic()) -
+        reinterpret_cast<const uint8_t*>(&detail::gGkAtoms);
+    _0 = (offset << 1) | 1;
+  } else {
+    _0 = reinterpret_cast<uintptr_t>(atom);
+  }
+  MOZ_ASSERT(IsStatic() == atom->IsStatic());
+  MOZ_ASSERT(AsAtom() == atom);
+}
+
 inline StyleAtom::StyleAtom(const StyleAtom& aOther) : _0(aOther._0) {
   if (!IsStatic()) {
     reinterpret_cast<nsAtom*>(_0)->AddRef();

servo/ports/geckolib/cbindgen.toml

@@ -466,6 +466,8 @@ renaming_overrides_prefixing = true
   inline bool IsStatic() const;
   inline nsAtom* AsAtom() const;
 
+  inline explicit StyleAtom(already_AddRefed<nsAtom> aAtom);
+
   // Could be implemented if wanted.
   StyleAtom& operator=(const StyleAtom&) = delete;