[Reporting] Reduce backoff policy when --run-web-tests

The reporting service delays reports by 60s. This gives the opportunity
to group several reports into a single request.

However, when running web_tests, we can't afford spending 60s. So this
is reduced to 100ms to avoid timeout.

So far, so good. The problem is about a second timer that used to be
proportional: |endpoint_backoff_policy.initial_delay_ms|. They are both
set to 60s. However in --run-web-tests, only the first is reduced to
100ms.

This patch reduced the second one to 100ms as well, to keep them always
proportional.

After this patch, the configuration becomes:

[Normal mode]
reporting_policy.delivery_interval = 60s;
reporting_policy.endpoint_backoff_policy.initial_delay = 60s

[web-tests mode]
reporting_policy.delivery_interval = 100ms;
reporting_policy.endpoint_backoff_policy.initial_delay = 100ms.

Bug: 1109194
Change-Id: I068a772e9eb95ec4c61466e290c400867ffbb63e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2316215


Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Lily Chen <chlily@chromium.org>
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791481}