Auto merge of #12233 - KiChjang:underline-color, r=emilio

Make text decorations have the same color as the text if no shadows are present

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #11870 (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [x] These changes do not require tests because according to @SimonSapin, making reftests against underlines are impossible

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tests/wpt/metadata-css/css21_dev/html4/painting-order-underline-001.htm.ini

@@ -1,3 +0,0 @@
-[painting-order-underline-001.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/text-decoration-va-length-001.htm.ini

@@ -1,3 +0,0 @@
-[text-decoration-va-length-001.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/text-decoration-va-length-002.htm.ini

@@ -1,3 +0,0 @@
-[text-decoration-va-length-002.htm]
-  type: reftest
-  expected: FAIL