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Adds feature flag WebUIThemeModeDoodles, which enables showing Doodles
on themed and dark mode NTPs. Furthermore, this CLs implements the dark
mode change protocol for interactive doodles. The protocol looks as
follows:

1. Append theme_messages=0 to the doodle URL. This tells the doodle that
   the mode change messaging protocol with version 0 is enabled.

2. Every time the mode changes postMessage
   {
     cmd: 'changeMode',
     dark: true|false,
   }
   to the doodle.

3. If the doodle postMessages
   {
     cmd: 'sendMode',
   }
   respond with the postMessage from 2. above. This is to work around
   a race condition where the NTP could postMessage the mode before the
   doodle has installed a message listener.

Bug: 688960
Change-Id: Icc1e333f33fd1f01a2c8d1d4a581fdf360ca7090
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2253220
Commit-Queue: Tibor Goldschwendt <tiborg@chromium.org>
Auto-Submit: Tibor Goldschwendt <tiborg@chromium.org>
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#780392}