Hotfix UAF when closing tab with sharing dialog.

This fixes a UAF when closing a tab while there is a
SharingDialog open.

(cherry picked from commit fd7196a)

Bug: 1000934
Change-Id: Ie8f6e52626ee834bc72acfe318c5a28aafe27635
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1789150
Commit-Queue: Alex Chau <alexchau@chromium.org>
Reviewed-by: Alex Chau <alexchau@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#694682}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1796242
Cr-Commit-Position: refs/branch-heads/3904@{chromium#43}
Cr-Branched-From: 675968a-refs/heads/master@{#693954}

chrome/browser/ui/views/sharing/sharing_dialog_view.cc

@@ -333,7 +333,8 @@ base::string16 SharingDialogView::GetWindowTitle() const {
 }
 
 void SharingDialogView::WindowClosing() {
-  controller_->OnDialogClosed(this);
+  if (web_contents())
+    controller_->OnDialogClosed(this);
 }
 
 // static