Spanify some variants of WTF::Equal()

This CL spanifies:

  `WTF::Equal(const StringImpl*, const LChar*, wtf_size_t)`
  `WTF::Equal(const StringImpl*, const UChar*, wtf_size_t)`
  `WTF::Equal(const StringImpl*, const char*, wtf_size_t)`

This CL should have no behavior changes.

Bug: 351564777
Change-Id: I79687da4aef66e91898b4c3d53fc8cfa3e5789d0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6022437
Auto-Submit: Kent Tamura <tkent@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Reviewed-by: Fredrik Söderquist <fs@opera.com>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1383276}