[@layer] Implement layer serialization in @import rules

[chromium-wpt-export-bot]

This patch also fixes a bug in ConsumeAtRulePrelude that the result
prelude may contain leading white spaces. The new test requires the fix
to pass.

Bug: 1095765
Change-Id: Ie565309d38aa208aa0e60b5c862ec1e62a5764b2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3252521
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/main@{#937046}

[wpt-pr-bot]

The review process for this patch is being conducted in the Chromium project.

[past]

Changes in this PR contain lint errors, listed below. These errors must either be fixed or added to the list of ignored errors; see the documentation.

css/css-cascade/parsing/layer-import.html: The filename layer-import in the css-cascade-5 testsuite is shared by: css/css-cascade/layer-import.html, css/css-cascade/parsing/layer-import.html (CSS-COLLIDING-TEST-NAME)
css/css-cascade/layer-import.html: The filename layer-import in the css-cascade-5 testsuite is shared by: css/css-cascade/layer-import.html, css/css-cascade/parsing/layer-import.html (CSS-COLLIDING-TEST-NAME)

@xiaochengh , @andruud, could you please take a look?

[past]

@tabatkins do you know if anything will break spectacularly if I admin-merge this despite the lint error?

[past]

Filed crbug.com/1268508 after offline discussion with Tab.