R1:55e2efc6fcc3

Update PHUIDocument to use new header gradients

Summary: Adds the new gradient to document views

Test Plan: Tested multiple pages in my sandbox in Phriction, UIExamples.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: Korvin, aran

Differential Revision: https://secure.phabricator.com/D6827