mac: use NSUserDefaults values for caret blink rate if present

The main use of these by end-users is to disable cursor blink by setting them
to extremely large values.

BUG=500168

Review-Url: https://codereview.chromium.org/2282533002
Cr-Commit-Position: refs/heads/master@{#414752}
4 files changed