[Zine] Fixing crash when SnippetsBridge is accessed after being destroyed

Replacing suggestion source with an empty suggestions source, once
SuggestionsUiDelegateImpl is destroyed.

Bug: 898078
Change-Id: Ic6f05da92e0c90f566275d4b9f0e1fb12afeb2a6
Reviewed-on: https://chromium-review.googlesource.com/c/1324611
Reviewed-by: Theresa <twellington@chromium.org>
Commit-Queue: Filip Gorski <fgorski@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606264}
1 file changed