[tasks] Cleanup OperationsBarrier

The class disallows copy construction, but still allows copy assignment.
This is fixed in this CL.

Drive-by: Fix punctuation.
Drive-by 2: Fix indentation in code example.

R=etiennep@chromium.org

Bug: v8:11074
Change-Id: I09e993a69d72e262d7b220200ef94b36d346548e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2584246Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71828}

src/tasks/operations-barrier.h

@@ -35,9 +35,7 @@ namespace internal {
 // }
 //
 // void MaybeRunOperation() {
-//   auto token = barrier_.TryLock();
-//   if (token)
-//     Process();
+//   if (token = barrier_.TryLock()) Process();
 // }
 //
 class V8_EXPORT_PRIVATE OperationsBarrier {
@@ -46,7 +44,7 @@ class V8_EXPORT_PRIVATE OperationsBarrier {
   // operation while being certain it happens-before CancelAndWait(). Releasing
   // this Token relinquishes this right.
   //
-  // This class is thread-safe
+  // This class is thread-safe.
   class Token {
    public:
     Token() = default;
@@ -54,11 +52,19 @@ class V8_EXPORT_PRIVATE OperationsBarrier {
       if (outer_) outer_->Release();
     }
     Token(const Token&) = delete;
-    Token(Token&& other) V8_NOEXCEPT {
-      this->outer_ = other.outer_;
+    Token(Token&& other) V8_NOEXCEPT : outer_(other.outer_) {
       other.outer_ = nullptr;
     }
 
+    Token& operator=(const Token&) = delete;
+    Token& operator=(Token&& other) V8_NOEXCEPT {
+      DCHECK_NE(this, &other);
+      if (outer_) outer_->Release();
+      outer_ = other.outer_;
+      other.outer_ = nullptr;
+      return *this;
+    }
+
     operator bool() const { return !!outer_; }
 
    private: