Use the whitespace flag in each glyph instead of going to the string for whitespace stripping

[jdm]

The PR in #8994 was designed to fix "str::slice_chars was O(n) and #2 in the profile." but it had test failures:

  ▶ FAIL [expected PASS] /css21_dev/html4/block-non-replaced-width-007.htm
  └   → /css21_dev/html4/block-non-replaced-width-007.htm eb195905e02d7943de70d2b8ac617e074179e996
/css21_dev/html4/reference/block-non-replaced-width-007-ref.htm 47ad8313feacfb8add7c5d1c014c7975027bf5b7
Testing eb195905e02d7943de70d2b8ac617e074179e996 == 47ad8313feacfb8add7c5d1c014c7975027bf5b7

  ▶ PASS [expected FAIL] /css21_dev/html4/floats-wrap-bfc-002-right-table.htm