Use ConsumeNumber() for flex shorthand

Remove use of ConsumeNumberRaw_DO_NO_USE which does not handle calc()-
expressions.

Bug: 405311304
Change-Id: I2a7ed41592239d43428eaf8dc8663d6c0d956710
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6404314
Reviewed-by: Morten Stenshorne <[email protected]>
Reviewed-by: Daniil Sakhapov <[email protected]>
Commit-Queue: Rune Lillesveen <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1439301}

Author: lilles

css/css-flexbox/flex-shorthand-calc.html

@@ -11,6 +11,6 @@
   test_shorthand_value('flex', 'sign(1em - 1px) sibling-index()', {
     'flex-grow': 'sign(1em - 1px)',
     'flex-shrink': 'sibling-index()',
-    'flex-basis': 'auto'
+    'flex-basis': '0%'
   });
 </script>