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Before crrev.com/c/1899420, bluez informed Chrome each time an
advertisement was received, even if nothing had changed about the
advertisement from its previous instance. Once that CL landed,
bluez switched to only informing Chrome of an advertisement when
it was first found during a scan, or when metadata of the
advertisement changed (i.e., RSSI, which can remain unchanged
for several seconds). This CL accounts for that new behavior
by informing observers an advertisement has been received on
the DeviceAdded() event.

I ran an Instant Tethering scan 50 times without this patch, and
50 times with it, and recorded time taken to find an expected BLE
advertisement (found in chrome://histograms):

* Without patch:
  * Mean:             4436 ms.
  * Median:           1680 ms.
  * 75th percentile: 11135 ms.
  * 95th percentile: 12185 ms.

* With patch:
  * Mean:              636 ms.
  * Median:            522 ms.
  * 75th percentile:   895 ms.
  * 95th percentile:  1838 ms.

Fixed: b:149945805,1051769
Change-Id: I73dc1d5678d166a6ce443ddfda76c34212aa92aa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2071377Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Miao-chen Chou <mcchou@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Ryan Hansberry <hansberry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746060}