Removing useless Win7 checks + standardize its use

For example, if the comment mention Win8+, the check
will be base::win::GetVersion() >= base::win::VERSION_WIN8.

BUG=579196

Review-Url: https://codereview.chromium.org/2909943003
Cr-Commit-Position: refs/heads/master@{#476823}
44 files changed