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This variable was never written, so we were always reading an uninitialized
value.  However, the only effect was to toggle between the UPPER and LOWER track
parts, which AFAIK always look the same.  So I don't think this could have a
user-visible effect.  Still, it was technically UB.

Fix it the right way by making ScrollBarViews paint the track in upper and lower
pieces and set the bit correctly for each one.  Thus, if the appearance actually
differs for any theme out there, this should get it right.

This also copies the parallel-meaning WebThemeEngine |is_back| member, so
scrollbars painted from that source have the bit set too.

Bug: 374410
Change-Id: Ib37f54e369f08dd6439d9edb969af13cc02ddf3b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1790474Reviewed-by: Robert Liao <robliao@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694540}