[automerger] Add extra (size_t) cast to avoid compiler warning. am: 1387a89b14 am: bc2ee79a6e am: 53787550a7 am: 95129dbdb6 am: 10d8009592 am: e04fe5b516 am: aca21c9df5 am: db203bf112 am: 14cd06a72d am: 2e47278b09 am: 48d106d6bf am: cd33bb4337 am: d31b8df am: 12c4073fa7 am: 0aab2c5ab8  -s ours am: 2352868d4d  -s ours
am: 9fe8755297  -s ours

Change-Id: I47a7548d08a9fc953142af10bf604690cdfb09e6