Change the data type of buffer memory sizes to 64 bit integers.

BUG=892288

Change-Id: I10e07b86f2ccc0cb61c8d7ee0d439e00d5eb8d3a
Reviewed-on: https://chromium-review.googlesource.com/c/1312244Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>