Support xywh() and rect() values for clip-path

This patch wires up the existing `xywh` and `rect` basic shape code
for use as a clip-path value. A followup will be needed to fix
clip-path-computed.html (see: https://crbug.com/1481082).

Chrome-status entry:
  https://chromestatus.com/feature/5672616710373376

Bug: 1481015
Change-Id: I0f4babf83873b3fec4ed9be943ca4e44780af824
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4851659
Reviewed-by: Fredrik Söderquist <fs@opera.com>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1195970}

css/css-masking/clip-path/clip-path-xywh-003.html

@@ -4,7 +4,7 @@
   <title>CSS Masking: Test clip-path property and xywh function</title>
   <link rel="help" href="https://drafts.csswg.org/css-shapes-1/#funcdef-basic-shape-xywh">
   <link rel="match" href="reference/clip-path-xywh-003-ref.html">
-  <meta name=fuzzy content="maxDifference=0-2;totalPixels=0-30">
+  <meta name=fuzzy content="maxDifference=0-2;totalPixels=0-32">
   <meta name="assert" content="The clip-path property takes the basic shape
 	'xywh()' for clipping. On pass you should see a green rect with round.">
 </head>