Make FontGlobalContext::GetHarfBuzzFontCache() to return a reference

This patch changes |FontGlobalContext::GetHarfBuzzFontCache()| to return
a reference instead of pointer to simplify source code for improving
code health.

Note: This CL is a preparation of the CL[1].

[1] http://crrev.com/c/3211416: Parallel Text Shaping

Bug: 1264280
Change-Id: Ic073c1ac28393a4c1d41ebb8c98eee64121e83c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3552069
Auto-Submit: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/main@{#985918}