[cjk-fonts] Use const ref instead of pointer in `font_fallback_win.cc`

To match the current Google C++ Style Guide[1].

This patch has no behavior changes.

[1] https://google.github.io/styleguide/cppguide.html#Inputs_and_Outputs

Bug: 343302583
Change-Id: Idfcbd4c3b020e84a0f6685fde105a12e12382380
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5619994


Auto-Submit: Koji Ishii <kojii@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1313705}