Use HighlightPainter::ComputeBackgroundRect some more

All the background rect paint instances in HighlightPainter::Paint can
use this function. Makes one PaintRect() overload unused. Also simplify
the function itself by leveraging a suitablee operator+ for
PhysicalRect.

Change-Id: I1f22cb6b9f9563af25048eba2048b1b32b1d244a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5335736


Reviewed-by: Stephen Chenney <schenney@chromium.org>
Auto-Submit: Fredrik Söderquist <fs@opera.com>
Commit-Queue: Stephen Chenney <schenney@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1267166}