Add a CreateEmpty() constructor to TabGroupId

This is useful for the extensions API, where it's helpful to have something like:

    tab_groups::TabGroupId group = tab_groups::TabGroupId::CreateEmpty();
    GetGroupFromId(id, &group);  // Populates group by reference, with an existing value.
    if (!group.is_empty()) {
      // Logic handling the group
    }

Having CreateEmpty() and is_empty() helps make this pattern intentional without accidentally creating empty groups and breaking assumptions in the tabstrip.

Change-Id: Ibb217edfd8ba599023488ea3d1a33277b49d8284
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2415573Reviewed-by: Charlene Yan <cyan@chromium.org>
Commit-Queue: Connie Wan <connily@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808028}

components/tab_groups/tab_group_id.cc

@@ -16,6 +16,11 @@ TabGroupId TabGroupId::FromRawToken(base::Token token) {
   return TabGroupId(token);
 }
 
+// static
+TabGroupId TabGroupId::CreateEmpty() {
+  return TabGroupId(base::Token());
+}
+
 TabGroupId::TabGroupId(const TabGroupId& other) = default;
 
 TabGroupId& TabGroupId::operator=(const TabGroupId& other) = default;

components/tab_groups/tab_group_id.h

@@ -18,6 +18,11 @@ class COMPONENT_EXPORT(TAB_GROUPS) TabGroupId {
   // call on a valid TabGroupId.
   static TabGroupId FromRawToken(base::Token token);
 
+  // Should only be used if intending to populate the TabGroupId by reference,
+  // using a valid existing ID. Primarily needed for the Tab Groups extensions
+  // API.
+  static TabGroupId CreateEmpty();
+
   TabGroupId(const TabGroupId& other);
 
   TabGroupId& operator=(const TabGroupId& other);
@@ -28,6 +33,8 @@ class COMPONENT_EXPORT(TAB_GROUPS) TabGroupId {
 
   const base::Token& token() const { return token_; }
 
+  bool is_empty() { return token_.is_zero(); }
+
   std::string ToString() const;
 
  private: