Handle sync changes on the Start Page tiles while context menus are showing.
Handle sync changes on the Start Page tiles while context menus are showing.

This handles the case where
SearchResultTileItemView::SetSearchResult(SearchResult*) may be called
from a nested run loop while its context menu is showing. This cancels
the menu (it's for the old item).

BUG=462470
TEST=Right-click on a start page app to get its context menu. On another
machine (with the same profile), uninstall the app. Context menu should
disappear. Chrome shouldn't crash.

Review URL: https://codereview.chromium.org/969953002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#318844}

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