The virtual tests for dark mode were scattered in virtual test
directory. This patch moves them in web_tests/dark-mode, a single real
directory. The dark mode has two main categories of the tests viz.
colors and images. The tests are now organized in these categories.

The web_tests/dark-mode runs tests in normal mode. The virtual tests
run the tests in dark mode with additional arguments.

Bug: 1116914

Change-Id: I5c8dfc9b6085e5d70475a82d018c25482f53fb6b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2377037
Commit-Queue: Prashant Nevase <prashant.n@samsung.com>
Reviewed-by: Stephen Chenney <schenney@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803204}