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This is a reland of e45a6ef0

The original CL was reverted because a DCHECK was being hit. The
TrayBluetoothHelperLegacy implementation would sometimes call
OnBluetoothSystemStateChanged without the state actually changing
which would cause the DCHECK to be hit.

A separate CL (https://crrev.com/c/1352123) fixed
TrayBluetoothHelperLegacy so now the DCHECK shouldn't be hit.

Original change's description:
> bluetooth: Query for devices every second in TrayBluetoothHelper
>
> Before TrayBluetoothHelper would get notified of any changes to devices.
> This caused problems in busy environments because TrayBluetoothHelper
> would get notified hundreds or even thousands of times per second of
> devices changes.
>
> This CL changes to a "pull" pattern where TrayBluetootHelper queries
> for devices every second. The devices are then cached so that clients
> of TrayBluetoothHelper can access them synchronously.
>
> TrayBluetoothHelper queries for devices as long as the state is
> kPoweredOn. If the state changes, the device list is cleared.
>
> Bug: 882346
>
> TEST=The following actions should be performed:
>
> 1. Turn bluetooth on and off in system tray.
> 2. Open device list in system menu.
> 3. Connect to a device.
> 4. See that the pod feature button in the system tray indicates a
>    device is connected when connected to a device.
>
> Change-Id: Id9028546a627f260527aa737f51c032f74a447ec
> Reviewed-on: https://chromium-review.googlesource.com/c/1347638
> Commit-Queue: Giovanni Ortuño Urquidi <ortuno@chromium.org>
> Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#611069}

TBR=tetsui@chromium.org

Bug: 882346
Change-Id: Ief9ce3d77a317dfde2a5fcc49cb0c598aaa9012d
Reviewed-on: https://chromium-review.googlesource.com/c/1352078Reviewed-by: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Commit-Queue: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611949}