Fix a -Wsign-compare error under GCC 4.9.2.
Fix a -Wsign-compare error under GCC 4.9.2.

BUG=

Review URL: https://codereview.chromium.org/1322693004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#30603}

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh