[Merge M73] [NTP] Refresh dark mode when system dark mode is changed

Refresh dark mode styling on system dark mode change. This allows the
NTP to update without needing to restart Chrome.

If dark mode is changed, the frontend is notified to refresh the
current theme.

Also add a dark mode delimiter to the MV tile icon URL. This allows
separate icons to be generated for dark/light mode respectively.

Bug: 925417
Change-Id: Ide1935a1cd019c9c3ff6709de690a92b1bfc4705
Reviewed-on: https://chromium-review.googlesource.com/c/1437894
Commit-Queue: Kristi Park <kristipark@chromium.org>
Reviewed-by: Kyle Milka <kmilka@chromium.org>
Reviewed-by: Chris Palmer <palmer@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#626770}(cherry picked from commit ff3cf95ea71dc2ccc8319b5cf4b9ba4662053a6e)
Reviewed-on: https://chromium-review.googlesource.com/c/1450398
Reviewed-by: Kristi Park <kristipark@chromium.org>
Cr-Commit-Position: refs/branch-heads/3683@{#125}
Cr-Branched-From: e51029943e0a38dd794b73caaf6373d5496ae783-refs/heads/master@{#625896}
11 files changed