[text blink period] Cache blink period instead of fetching from defaults

When switching tabs the Views UI makes several calls to retrieve the
insertion point blink period, and each of these calls generates three
NSUserDefaults calls. In the old days NSUserDefaults simply manipulated
files on disk but now there's an XPC service that manages everything.
As a result, when switching tabs the browser spends 7% of its time
retrieving the blink period. This cl caches the blink period, forcing
updates when the user switches away and back to Chrome.

Bug: 1290921

Test: out/Default/ui_base_unittests --gtest_filter=DefaultsUtilsTest*
Change-Id: I568eb9ae3acd8e17971767a564a94613d0a9c980
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3419059
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Jayson Adams <shrike@chromium.org>
Cr-Commit-Position: refs/heads/main@{#964786}