Add base::UnguessableToken::Null() helper.

Provide a global constant for a null unguessable token, for functions
that need to return a const base::UnguessableToken&.

Change-Id: Ib07dde7862dcab19baac409eaecda3482cfc5f4d
Reviewed-on: https://chromium-review.googlesource.com/1162986Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580761}

base/unguessable_token.cc

@@ -5,6 +5,7 @@
 #include "base/unguessable_token.h"
 
 #include "base/format_macros.h"
+#include "base/no_destructor.h"
 #include "base/rand_util.h"
 #include "base/strings/stringprintf.h"
 
@@ -26,6 +27,12 @@ UnguessableToken UnguessableToken::Create() {
   return token;
 }
 
+// static
+const UnguessableToken& UnguessableToken::Null() {
+  static const NoDestructor<UnguessableToken> null_token;
+  return *null_token;
+}
+
 // static
 UnguessableToken UnguessableToken::Deserialize(uint64_t high, uint64_t low) {
   // Receiving a zeroed out UnguessableToken from another process means that it

base/unguessable_token.h

@@ -42,6 +42,12 @@ class BASE_EXPORT UnguessableToken {
   // Create a unique UnguessableToken.
   static UnguessableToken Create();
 
+  // Returns a reference to a global null UnguessableToken. This should only be
+  // used for functions that need to return a reference to an UnguessableToken,
+  // and should not be used as a general-purpose substitute for invoking the
+  // default constructor.
+  static const UnguessableToken& Null();
+
   // Return a UnguessableToken built from the high/low bytes provided.
   // It should only be used in deserialization scenarios.
   //