GitLab

The buffer_id is passed through Mojo as uint64, so we should refer to
the buffer_id throughout the code consistently as uint64_t.

BUG=832772
TBR=wuchengli

Change-Id: Ib6f260c4242e8c68510903cefcc997bf7a6bbd4d
Reviewed-on: https://chromium-review.googlesource.com/1073213Reviewed-by: Ricky Liang <jcliang@chromium.org>
Commit-Queue: Ricky Liang <jcliang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561854}