Fix incorrect usage of percentage unit in tab-size test

This CL changes the incorrect use of `word-spacing: 100%` to `1ch` in
the `tab-size-spacing-003.html` test.

In CSS Text Level 4, percentage values for word-spacing are defined to
resolve against the computed font-size of the element, not the advance
width of the space character. Meanwhile, tab-size is defined as a
multiple of the advance width of U+0020.

As a result, comparing tab-size alignment to spacing derived from
`word-spacing: 100%` mixes unrelated units and leads to an invalid test
condition.

This patch replaces the percentage value with `1ch` to preserve the test
intent while ensuring spec compliance.

Specs:
- https://www.w3.org/TR/css-text-4/#word-spacing-property
- https://www.w3.org/TR/css-text-3/#tab-size-property

Bug: 41435273
Change-Id: If1564727af832210af495096bd46786b80969f29
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6815996
Reviewed-by: Kent Tamura <[email protected]>
Reviewed-by: Koji Ishii <[email protected]>
Commit-Queue: Minseong Kim <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1498615}

Author: jja08111

css/css-text/tab-size/tab-size-spacing-003.html

@@ -12,11 +12,11 @@
   white-space: pre;
 }
 .test {
-  word-spacing: 100%; /* effectively double the size of SPACE, and hence of tabs */
+  word-spacing: 1ch; /* effectively double the size of SPACE, and hence of tabs */
   tab-size: 3;
 }
 .ref1 {
-  word-spacing: 100%;
+  word-spacing: 1ch;
 }
 .ref2 {
   white-space: pre;