[Extensions Cleanup] Make ExtensionPrefs use a base::Clock

For some reason, ExtensionPrefs has a custom TimeProvider class to
provide the current time (which is overridden in testing). Instead,
use the more standard base::Clock class.

Bug: None
Change-Id: I7309fdf3a003f02cedbf20b470b3031831dcce5a
Reviewed-on: https://chromium-review.googlesource.com/676267
Reviewed-by: Karan Bhatia <[email protected]>
Commit-Queue: Devlin <[email protected]>
Cr-Commit-Position: refs/heads/master@{#503321}
3 files changed