Fix ReducedMemoryUsage for OomIntervention

Currently ReducedMemoryUsage metrics in OomIntervention reports negative
numbers when intervention reduces memory. This is confusing and thus
this CL renames the metrics.

Bug: 939702
Change-Id: I5bd7e8ba4d7ffb682ea4565a39964cce74b84c67
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1510704
Commit-Queue: Yuzu Saijo <yuzus@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Reviewed-by: Keishi Hattori <keishi@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#639824}
3 files changed