Rename histogram MappedAndResidentMemoryFootprint2

The device coverage where this histogram is recorded successfully has
significantly increased. To avoid bias from previous Chrome versions,
rename this histogram.

Also fix a typo.

Bug: 1070618
Change-Id: Id7d4cc1b49d90122a75b4c6bd2e69093b53e162e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2435186


Reviewed-by: ssid <ssid@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Egor Pasko <pasko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812008}