Correct misleading comments on RefCountedThreadSafe

In the previous CL: https://codereview.chromium.org/2747673002,
"RefCountedThreadSafe" has been used again but the comments were
not updated and misleading. And RefCountedThreadSafe does not
have virtual functions now.

BUG=None
Test=no test

Change-Id: I5d9a31a2d0f3d62af8772813db4d34e7f725773d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1742257Reviewed-by: Albert J. Wong <ajwong@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Commit-Queue: Honglin Yu <honglinyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#685441}

base/callback_internal.h

@@ -45,11 +45,10 @@ using PassingType = std::conditional_t<std::is_scalar<T>::value, T, T&&>;
 // DoInvoke function to perform the function execution.  This allows
 // us to shield the Callback class from the types of the bound argument via
 // "type erasure."
-// At the base level, the only task is to add reference counting data. Don't use
-// RefCountedThreadSafe since it requires the destructor to be a virtual method.
-// Creating a vtable for every BindState template instantiation results in a lot
-// of bloat. Its only task is to call the destructor which can be done with a
-// function pointer.
+// At the base level, the only task is to add reference counting data. Avoid
+// using or inheriting any virtual functions. Creating a vtable for every
+// BindState template instantiation results in a lot of bloat. Its only task is
+// to call the destructor which can be done with a function pointer.
 class BASE_EXPORT BindStateBase
     : public RefCountedThreadSafe<BindStateBase, BindStateBaseRefCountTraits> {
  public: