Fix OomIntervention.Reduced* metrics

The code calculated the deltas with uint64_t, causing negative values to be reported incorrectly.
This CL fixes the subtraction. And also clamps the samples to 1000 buckets (-500MB to 500MB).

Bug: 917293
Change-Id: Ie2b48ddbf1a47577692c4c2013c7a9d365c786ea
Reviewed-on: https://chromium-review.googlesource.com/c/1488475
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Keishi Hattori <keishi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636678}
4 files changed