This CL
* Uses entry colors instead of window colors for the NTP
* Fixes a leak involving ScopedGObject
* Simplifies how background and border colors are calculated
* Adds the ".label" class only on versions of Gtk before 3.20
* Fixes text selection colors on one theme
* Updates the doc to reflect the new way colors are calculated

BUG=132847

Review-Url: https://codereview.chromium.org/2683953005
Cr-Commit-Position: refs/heads/master@{#449709}