[Nearby] Prevent crash on potential double close of sharesheet.

This clears the controller in the OnClose() method to prevent a crash
in the event that the OnClose() callback runs more than once before the
animation is complete. This surfaced as a crash when sending a file.

Fixed: 1150176
Change-Id: I13aa8eaf3bf3f5e382d31376825f647ab287a7dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2548373
Commit-Queue: Michael Hansen <hansenmichael@google.com>
Reviewed-by: James Vecore <vecore@google.com>
Cr-Commit-Position: refs/heads/master@{#829050}

chrome/browser/nearby_sharing/sharesheet/nearby_share_action.cc

@@ -119,6 +119,10 @@ void NearbyShareAction::OnClose() {
   // The nearby WebUI requested to close through user action
   if (controller_) {
     controller_->CloseSharesheet();
+
+    // We need to clear out the controller here to protect against calling
+    // CloseShareSheet() more than once, which will cause a crash.
+    controller_ = nullptr;
   }
 }