Cache ::selection:window-inactive

The non-standard :window-inactive allows ::selection style to be
different for active and inactive windows in Blink. Since we only had
one pseudo element type to identify style type in the pseudo style
cache, we used a non-caching version for the inactive case.

Instead, introduce an internal pseudo id for inactive selections such
that we can cache for inactive windows as well.

Change-Id: I878d729c462de7846157691b43b242597db07221
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2983277


Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#896046}