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Previously ash would make a direct connection to Chrome (content_browser) to
handle showing system tray menu UI. This is a layering violation and makes
it more difficult to run ash without chrome.

Chrome's SystemTrayClient now explicitly registers itself with ash on startup.
It no longer attempts to reconnect on error; my understanding is that chrome
tolerating an ash crash is not a goal.

BUG=none
TEST=manual, can still open settings via system tray menu

Review-Url: https://codereview.chromium.org/2525813004
Cr-Commit-Position: refs/heads/master@{#434736}