view_manager: fix crash when input events arrive early
view_manager: fix crash when input events arrive early

It's possible for input events to flow through the connection_manager early from
the platform_window before a connection is established. In this CL, if we can't
find a connection, then we simply drop the input event.

BUG=none

Review URL: https://codereview.chromium.org/1167373002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#333391}

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