Dark Mode: add WebUI handlers for querying/observing theme

To make your page track dark mode, do something like:

  #include "chrome/browser/ui/webui/dark_mode_handler.h"
  ...
  DarkModeHandler::Initialize(web_ui, data_source);

R=dpapad@chromium.org
BUG=883049

Change-Id: Ic58478513c3917f663cb29436cf79a3d59d53470
Reviewed-on: https://chromium-review.googlesource.com/c/1359456
Commit-Queue: Dan Beam <dbeam@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613682}
7 files changed