Update comment for SkFilterQuality

SkFilterQuality is recorded in the chrome UMA metrics, the enum should
not be reordered or changed; otherwise, the result may produce false
alarm. Adding the comment to reflect it t avoid future misunderstanding.

Change-Id: I6cdf0d8c4f59a5548e456a2c641b2b6158abde48
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/374417
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
1 file changed