Fix enum vs unsigned -Wsign-compare warning

Recent Clang versions started taking into account that enums are signed
on Windows when emitting these warnings.

Bug: 792519
Change-Id: I32782dc34fc0480942038e4950266448e38d2eec
Reviewed-on: https://chromium-review.googlesource.com/813495
Reviewed-by: Jungshik Shin <jshin@chromium.org>
3 files changed