Bug 1563065: Use const& in mozilla::Result constructors. r=froydnj

Change the `ResultImplementation` and `Result` constructors to accept success
values by `const` reference. This makes it possible for `Result` to carry
`MOZ_NON_PARAM` types as success values.

Differential Revision: https://phabricator.services.mozilla.com/D36682

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

mfbt/Result.h

@@ -47,7 +47,7 @@ class ResultImplementation<V, E, PackingStrategy::Variant> {
   mozilla::Variant<V, E> mStorage;
 
  public:
-  explicit ResultImplementation(V aValue) : mStorage(aValue) {}
+  explicit ResultImplementation(const V& aValue) : mStorage(aValue) {}
   explicit ResultImplementation(E aErrorValue) : mStorage(aErrorValue) {}
 
   bool isOk() const { return mStorage.template is<V>(); }
@@ -68,7 +68,7 @@ class ResultImplementation<V, E&, PackingStrategy::Variant> {
   mozilla::Variant<V, E*> mStorage;
 
  public:
-  explicit ResultImplementation(V aValue) : mStorage(aValue) {}
+  explicit ResultImplementation(const V& aValue) : mStorage(aValue) {}
   explicit ResultImplementation(E& aErrorValue) : mStorage(&aErrorValue) {}
 
   bool isOk() const { return mStorage.template is<V>(); }
@@ -300,7 +300,7 @@ class MOZ_MUST_USE_TYPE Result final {
   /**
    * Create a success result.
    */
-  MOZ_IMPLICIT Result(V aValue) : mImpl(aValue) { MOZ_ASSERT(isOk()); }
+  MOZ_IMPLICIT Result(const V& aValue) : mImpl(aValue) { MOZ_ASSERT(isOk()); }
 
   /**
    * Create an error result.