Migrate WaitableEvent to enum-based constructor in base/

Change automated with clang-tidy (details @ https://crbug.com/612843#c13)

BUG=612843

Review-Url: https://codereview.chromium.org/2032603002
Cr-Commit-Position: refs/heads/master@{#397244}
38 files changed