[base] Remove unnecessary 'mutable' from WeakReferenceOwner.

Change-Id: I835c23132ddf9eaec25c6e8e49c3a14096340b34
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1906883
Auto-Submit: Wez <wez@chromium.org>
Reviewed-by: Albert J. Wong <ajwong@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714778}

base/memory/weak_ptr.h

@@ -140,7 +140,7 @@ class BASE_EXPORT WeakReferenceOwner {
   void Invalidate();
 
  private:
-  mutable scoped_refptr<WeakReference::Flag> flag_;
+  scoped_refptr<WeakReference::Flag> flag_;
 };
 
 // This class simplifies the implementation of WeakPtr's type conversion