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To ensure that ARC would consistently get IP properties from the host
for all networks, change-id I24d26d6492f133e0c38949c8c3b4403687bfac21
(https://chromium-review.googlesource.com/c/chromium/src/+/2028881)
added a delayed property update request to shill when ArcNetHostImpl
cannot retrieve IP properties for a Shill connected Service.

However, one such request will trigger multiple property updates from
Shill and will retrigger the same codepath in ArcNetHostImpl that asked
for the property update request. This amplification can lead to an
infinite loop between Chrome and Shill incurring high CPU usage on the
system when ArcNetHostImpl will keep failing to find the IP properties
from the ONC properties or from the Device cached state.

This patch prevents this loop from happening at a high frequency by
making sure that at most one delayed property update request is
scheduled. A follow-up patch will add exponential backoff for the retry
delays.

It was reported in crbug/1054676 to happen consistently for the case of
native VPNs backed by a WiFi connection (i.e it does not happen with
VPNs backed by ethernet). The reason why ArcNetHostImpl fails to find IP
properties in that specific scenario is still unknown and will be
investigated separately.

BUG=1054676
TEST=Compiled, connected L2TP VPNs, observed that logs patterns seen in
crbug/1054676 do not happen and that cpu% of shill, chronos, and
system_server stays at normal levels.

Change-Id: I2a39ba9c8d14cf42b6b0dbdd10c885bf5be09cfa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2078372
Commit-Queue: Hugo Benichi <hugobenichi@google.com>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746192}