Ship prefers-color-scheme media query.

Set the flag to 'stable' and fix test expectations.

Intent-to-ship:

https://groups.google.com/a/chromium.org/d/topic/blink-dev/sLK1cLgvieg/discussion

Bug: 889087
Change-Id: I235429a514af0acc6f1e991c6f38cb52ca0fd41e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1593378Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#656331}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -824,6 +824,7 @@
     },
     {
       name: "MediaQueryPrefersColorScheme",
+      status: "stable",
     },
     {
       name: "MediaQueryShape",

third_party/blink/web_tests/external/wpt/css/mediaqueries/prefers-color-scheme-expected.txt

-This is a testharness.js-based test.
-PASS Should be parseable in a CSS stylesheet: '(prefers-color-scheme)'
-FAIL Should be parseable in a CSS stylesheet: '(prefers-color-scheme: no-preference)' assert_true: expected true got false
-FAIL Should be parseable in a CSS stylesheet: '(prefers-color-scheme: light)' assert_true: expected true got false
-FAIL Should be parseable in a CSS stylesheet: '(prefers-color-scheme: dark)' assert_true: expected true got false
-PASS Should not be parseable in a CSS stylesheet: '(prefers-color-scheme: 0)'
-PASS Should not be parseable in a CSS stylesheet: '(prefers-color-scheme: none)'
-PASS Should not be parseable in a CSS stylesheet: '(prefers-color-scheme: 10px)'
-PASS Should not be parseable in a CSS stylesheet: '(prefers-color-scheme: dark 0)'
-PASS Should not be parseable in a CSS stylesheet: '(prefers-color-scheme: dark light)'
-PASS Should not be parseable in a CSS stylesheet: '(prefers-color-scheme: light/dark)'
-PASS Should be parseable in JS: '(prefers-color-scheme)'
-FAIL Should be parseable in JS: '(prefers-color-scheme: no-preference)' assert_true: expected true got false
-FAIL Should be parseable in JS: '(prefers-color-scheme: light)' assert_true: expected true got false
-FAIL Should be parseable in JS: '(prefers-color-scheme: dark)' assert_true: expected true got false
-PASS Should not be parseable in JS: '(prefers-color-scheme: 0)'
-PASS Should not be parseable in JS: '(prefers-color-scheme: none)'
-PASS Should not be parseable in JS: '(prefers-color-scheme: 10px)'
-PASS Should not be parseable in JS: '(prefers-color-scheme: dark 0)'
-PASS Should not be parseable in JS: '(prefers-color-scheme: dark light)'
-PASS Should not be parseable in JS: '(prefers-color-scheme: light/dark)'
-FAIL Check that no-preference evaluates to false in the boolean context assert_equals: expected true but got false
-Harness: the test ran to completion.
-