[Telemetry] Move test.DidRunTest call to shared_page_state.TearDown().

The main motivation is to remove the usage of browser instance in page_runner
(along with https://codereview.chromium.org/737463002/).

One behavior change to notice is that the original implementation calls
DidRunTest exactly once, but this patch may make DidRunTest called multiple
times.

There are 3 perf tests that overrides page_test.DidRunTest:
dom_perf.DidRunTest: which doesn't touch browser.
peacekeeper.DidRunTest: which doesn't touch browser.
page_cycler.DidRunTest: use browser to get the IO metric.

The reason why page test has DidRunTest hook is that it provides the way to
summarize statistics for a *single browser instance* across many tests. If
TearDown is called the second time, the browser must be restarted hence makes
the DidRunTest's computation meaningless. This scenario does create test
failures, so the dashboard should be adjusted to ignore the summary metrics in
case this failure happens.

BUG=418278

Review URL: https://codereview.chromium.org/721443005

Cr-Commit-Position: refs/heads/master@{#304515}