Make sure we have non-zero idle time even after rounding to size_t

BUG=v8:3728
R=machenbach@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/764263002

Cr-Commit-Position: refs/heads/master@{#25578}
Fix clang++ warnings on -Wtautological-constant-out-of-range-compare

BUG=v8:3730
LOG=N
R=dslomov@chromium.org

Review URL: https://codereview.chromium.org/750093003

Cr-Commit-Position: refs/heads/master@{#25579}