DevTools: Ensure to fetch contrast for the right element in highlight.

In NodeHighlightTool ctor, we may override which node is actually
used, if the node in question is under a content-visibility element
which prevents visibility. When later on we fetch the contrast values
for that node, we should use |node_| as opposed to |node| since at
that time, the two values could be different.

Note that this changes includes leveraging NodeHighlightTool directly
in the getHighlightObjectForTest function, since previously the two
implementations were different. The updated test would now fail without
the |node| -> |node_| switch, but passes with the switch.

R=chrishtr@chromium.org

Fixed: 1090371
Change-Id: Ic98aafaf02818203311bf6a745575a0612612914
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2228124
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: vmpstr <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#775172}