[Sharesheet] Add nullcheck before callback.

We are adding a nullcheck before the callback because callers
may call ShowBubble with NullCallback.

Bug: 1142706
Change-Id: I2d3c1376b036057a2e5890e20a10f6478d5c7075
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2589139
Commit-Queue: Melissa Zhang <melzhang@chromium.org>
Auto-Submit: Melissa Zhang <melzhang@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836527}

chrome/browser/sharesheet/sharesheet_service_delegate.cc

@@ -31,7 +31,9 @@ void SharesheetServiceDelegate::ShowBubble(
     sharesheet::CloseCallback close_callback) {
   if (is_bubble_open_) {
     // TODO(melzhang@) Update by adding SharesheetAlreadyOpenCancel.
-    std::move(close_callback).Run(sharesheet::SharesheetResult::kCancel);
+    if (close_callback) {
+      std::move(close_callback).Run(sharesheet::SharesheetResult::kCancel);
+    }
     return;
   }
   sharesheet_bubble_view_->ShowBubble(std::move(targets), std::move(intent),