Called before a context menu is displayed. |params| provides information
about the context menu state. |model| initially contains the default
context menu. The |model| can be cleared to show no context menu or
modified to show a custom menu. Do not keep references to |params| or
|model| outside of this callback.
Called before a context menu is displayed. |params| provides information about the context menu state. |model| initially contains the default context menu. The |model| can be cleared to show no context menu or modified to show a custom menu. Do not keep references to |params| or |model| outside of this callback.