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This patch adds vrdisplayconnect and vrdisplaydisconnect event support
of google vr sdk for android. It defines 'connect' status as
nativeContext is available and defines 'disconnect' status as
nativeContext is unavailable.

Note that connect event may dispatch before user get vr devices
because devices in connection status could already deliever user some device
info and user could use this event to do some preparing work.

BUG=389343
R=bajones@chromium.org, kenrd@chromium.org, ochang@chromium.org

Review-Url: https://codereview.chromium.org/2317483002
Cr-Commit-Position: refs/heads/master@{#419392}