DevTools: Front-end for platform font inspection

This patch adds UI for the 'platform font inspection' feature.

BUG=135489

Review URL: https://chromiumcodereview.appspot.com/22875023

git-svn-id: svn://svn.chromium.org/blink/trunk@156700 bbb929c8-8fbe-4397-9dbb-9b2b20218538