Remove ThemeColor runtime feature

According to Bug 383941, this feature has been stable since mid July.

BUG=402536

Review URL: https://codereview.chromium.org/684913002

git-svn-id: svn://svn.chromium.org/blink/trunk@184902 bbb929c8-8fbe-4397-9dbb-9b2b20218538