v8binding: Stops using the copyable ver of v8::Persistent. (reland)

The first attempt: https://crrev.com/c/798820
Its revert: https://crrev.com/c/802715
The cause: Forgot to v8::Persistent::Reset in the dtor.

I learnt that v8::Persistent provides two variations of copying.
One actually copies, and the other actually shares the underlying
slot.

This patch changes CallbackFunctionBase::Persistent from the
"share" version to the "copy" (= make another independent slot)
version.

Bug: 779036
Change-Id: Id088dfa5ac111dbcb314ee992c6ba626e3a6f8ce
Reviewed-on: https://chromium-review.googlesource.com/802609Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520912}

third_party/WebKit/Source/platform/bindings/CallbackFunctionBase.h

@@ -75,11 +75,24 @@ class CallbackFunctionBase::Persistent
     if (raw)
       function_.Reset(raw->GetIsolate(), raw->callback_function_.Get());
   }
-  Persistent(const Persistent& other)
-      : Parent(other), function_(other.function_) {}
-  ~Persistent() = default;
-
-  Persistent& operator=(const Persistent& other) = default;
+  Persistent(const Persistent& other) : Parent(other) {
+    if (other)
+      function_.Reset(other->GetIsolate(), other.function_);
+  }
+  ~Persistent() { function_.Reset(); }
+
+  Persistent& operator=(const Persistent& other) {
+    if (this == &other)
+      return *this;
+
+    if (other) {
+      Parent::operator=(other);
+      function_.Reset(other->GetIsolate(), other.function_);
+    } else {
+      Clear();
+    }
+    return *this;
+  }
 
   void Clear() {
     Parent::Clear();
@@ -87,8 +100,7 @@ class CallbackFunctionBase::Persistent
   }
 
  private:
-  v8::Persistent<v8::Function, v8::CopyablePersistentTraits<v8::Function>>
-      function_;
+  v8::Persistent<v8::Function> function_;
 };
 
 template <typename T>