cleanup TODOs, and warnings

The API for a remote glyph cache had a couple of TODO's left waiting
for Chromium to implement the functions. This happened three years
ago.

This is waiting for the Chromium CL to finish:
https://chromium-review.googlesource.com/c/chromium/src/+/3338117

Change-Id: If8e3de619f0f4be7044e6955204b599c5a42e802
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/484437
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Herb Derby <herb@google.com>
3 files changed