Auto merge of #16640 - hiikezoe:timing-function, r=xidorn

Use FunctionKeyword for computed_value of timing function

<!-- Please describe your changes on the following line: -->
https://bugzilla.mozilla.org/show_bug.cgi?id=1358330

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<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #15086

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- [X] These changes do not require tests because mozilla-central has test cases.  There might be some test cases in servo tree.

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tests/wpt/metadata-css/css-transitions-1_dev/html/transition-001.htm.ini

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+[transition-001.htm]
+  type: testharness
+  [parse '1s']
+    expected: FAIL
+
+  [parse '1s 2s']
+    expected: FAIL
+
+  [parse '1s 2s ease-in']
+    expected: FAIL
+
+  [parse '1s ease-in 2s']
+    expected: FAIL
+
+  [parse 'ease-in 1s 2s']
+    expected: FAIL
+
+  [parse '1s width']
+    expected: FAIL
+
+  [parse 'width 1s']
+    expected: FAIL
+
+  [parse '1s width 2s']
+    expected: FAIL
+
+  [parse '1s 2s width ease-in']
+    expected: FAIL
+
+  [parse '1s ease-in 2s width']
+    expected: FAIL
+
+  [parse 'width ease-in 1s 2s']
+    expected: FAIL
+
+  [parse 'width .1s ease-in .2s']
+    expected: FAIL
+

tests/wpt/metadata-css/css-transitions-1_dev/html/transition-timing-function-001.htm.ini

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+[transition-timing-function-001.htm]
+  type: testharness
+  [parse 'ease']
+    expected: FAIL
+
+  [parse 'linear']
+    expected: FAIL
+
+  [parse 'ease-in']
+    expected: FAIL
+
+  [parse 'ease-out']
+    expected: FAIL
+
+  [parse 'ease-in-out']
+    expected: FAIL
+
+  [parse 'cubic-bezier(foobar)']
+    expected: FAIL
+
+  [parse 'steps(foobar)']
+    expected: FAIL
+
+  [parse 'steps(3.3, end)']
+    expected: FAIL
+
+  [parse 'steps(3, top)']
+    expected: FAIL
+
+  [parse 'steps(-3, top)']
+    expected: FAIL
+
+  [parse 'cubic-bezier(-0.1, -0.2, -0.3, -0.4)']
+    expected: FAIL
+
+  [parse 'cubic-bezier(1.1, 1.2, 1.3, 1.4)']
+    expected: FAIL
+