StatsValue - define operator+=

while this is a little bit more code,
it seems much better for the accumulation operation
to be next to the struct definition itself
(in case we ever add more fields)

Test: TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I26022db4566e69c964298d7b3f2cc4fa4a9a5152
2 files changed