GitLab

Change the flow so that Chrome explicitly asks ash to show the icon
rather than having ash ask Chrome whether there is an update available.

* Add ShowUpdateIcon to system_tray.mojom
* Introduce update.mojom
* Migrate ash to using ash::mojom::UpdateSeverity internally
* Migrate update methods from SystemTrayDelegate to SystemTrayClient
* Add a new SystemTrayClientTest and move existing Flash test there

Also add docs for SystemTrayItem (used by TrayUpdate) since tray view
vs. default view vs. detailed view always confuses me.

BUG=647412
TEST=chrome browser_tests, ash_unittests

Review-Url: https://codereview.chromium.org/2558043006
Cr-Commit-Position: refs/heads/master@{#438051}