[Structured metrics] Fix bug in metrics upload

We cache soon-to-be-uploaded metrics on-disk. The logic for uploading
metrics is missing a call to delete that cache, which means that if the
following situation happens, the metrics will be uploaded twice:

1. some metrics are logged
2. those metrics are uploaded
3. the user logs out before any more metrics are logged

This is reasonably rare given what we log, but affected M90. This CL
fixes the issue.

(cherry picked from commit 6a20d97)

Bug: 1028447
Change-Id: Iffee3238050e33d6b5067638a02831d07a8516a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2814133
Reviewed-by: Thanh Nguyen <thanhdng@chromium.org>
Commit-Queue: Tony Yeoman <tby@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#870855}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2821678
Cr-Commit-Position: refs/branch-heads/4472@{#24}
Cr-Branched-From: 3d60439-refs/heads/master@{#870763}

components/metrics/structured/structured_metrics_provider.cc

@@ -231,6 +231,7 @@ void StructuredMetricsProvider::ProvideCurrentSessionData(
   structured_data->mutable_events()->Swap(
       events_.get()->get()->mutable_uma_events());
   events_.get()->get()->clear_uma_events();
+  events_->StartWrite();
 }
 
 bool StructuredMetricsProvider::HasIndependentMetrics() {