[css-pseudo] Fix marker-text-combine-upright.html

This test was failing for 2 reasons:
 1. ::marker had `text-align: start` and `text-align-last: start` as
    important UA rules, while the style adjuster only enforces
    `text-align: center` in the presence of `text-combine-upright: all`.
    This resulted in `text-align: center; text-align-last: start`.

    Arguably the style adjuster should also set `text-align-last`,
    but a smaller fix is changing the important UA rule for ::marker
    to `text-align-last: auto`. Note `text-align-last: auto` behaves as
    `text-align-last: start` when `text-align: start`, so the change
    has no effect in the great majority of cases.

 2. The style adjuster enforces `white-space: pre` on outside markers,
    even if the test uses `all: unset`.

    So this changes the test reference to expect `white-space: pre`,
    and explicitly sets `white-space: pre` in the test for Firefox.

Bug: 1060007

TEST=external/wpt/css/css-pseudo/marker-text-combine-upright.html

Change-Id: I06e6ac4034abadf956b9d7194b945c9a2c338542
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6021669
Commit-Queue: Oriol Brufau <obrufau@igalia.com>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1386262}

css/css-pseudo/marker-text-combine-upright-ref.html

@@ -19,6 +19,7 @@
   text-align: end;
 }
 span {
+  white-space: pre;
   text-combine-upright: all;
 }
 </style>

css/css-pseudo/marker-text-combine-upright.html

@@ -26,7 +26,12 @@
   content: "3. ";
 }
 ::marker {
+  /* Try to undo whatever styles browsers might have set on ::marker. */
   all: unset;
+  /* However, Blink still enforces `white-space: pre` on markers with outside position.
+     As a workaround, the reference expects `white-space: pre` too, but then we need
+     to set it here for Firefox. */
+  white-space: pre;
   text-combine-upright: all;
 }
 </style>