[OTS] build failure on android_aosp
[OTS] build failure on android_aosp

AOSP's compiler is too strict about signed and unsigned integer
comparison. Add a cast to fix the sign-compare warning.

TEST=compile
R=yusukes@chromium.org

Review URL: https://codereview.chromium.org/77173003

git-svn-id: 0e3d3500f59afd9abff519869ed878e5f2d4e9ab@107 a4e77c2c-9104-11de-800e-5b313e0d2bf3

android-ia/platform_external_chromium_org_third_party_ots,xin3liang/platform_external_chromium_org_third_party_ots,TeamEOS/external_chromium_org_third_party_ots,AOSPU/external_chromium_org_third_party_ots,geekboxzone/lollipop_external_chromium_org_third_party_ots,geekboxzone/lollipop_external_chromium_org_third_party_ots,bpsinc-native/src_third_party_ots,TeamEOS/external_chromium_org_third_party_ots,jgcaaprom/android_external_chromium_org_third_party_ots,android-ia/platform_external_chromium_org_third_party_ots,xin3liang/platform_external_chromium_org_third_party_ots,TeamEOS/external_chromium_org_third_party_ots,jgcaaprom/android_external_chromium_org_third_party_ots,AOSPU/external_chromium_org_third_party_ots,AOSPU/external_chromium_org_third_party_ots,xin3liang/platform_external_chromium_org_third_party_ots,Omegaphora/external_chromium_org_third_party_ots,geekboxzone/lollipop_external_chromium_org_third_party_ots,bpsinc-native/src_third_party_ots,AOSPU/external_chromium_org_third_party_ots,yinquan529/platform-external-chromium_org-third_party-ots,yinquan529/platform-external-chromium_org-third_party-ots,Omegaphora/external_chromium_org_third_party_ots,bpsinc-native/src_third_party_ots,xin3liang/platform_external_chromium_org_third_party_ots,android-ia/platform_external_chromium_org_third_party_ots,Omegaphora/external_chromium_org_third_party_ots,jgcaaprom/android_external_chromium_org_third_party_ots,yinquan529/platform-external-chromium_org-third_party-ots