Add virtual destructor in SharingDialog

To avoid potential memory leak in the future, we make the destructor
overridable.

Bug: 1029505
Change-Id: I75ac34ceface5a09404bb720528faab5e63b0f14
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1945819Reviewed-by: Richard Knoll <knollr@chromium.org>
Commit-Queue: Himanshu Jaju <himanshujaju@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720911}

chrome/browser/sharing/sharing_dialog.h

@@ -9,6 +9,8 @@
 // This object is responsible for its own lifetime.
 class SharingDialog {
  public:
+  virtual ~SharingDialog() = default;
+
   // Called to close the dialog and prevent future callbacks into the
   // controller.
   virtual void Hide() = 0;