Update Skia milestone to 57

Milestone that we typically update, but note that change from 55->56 was missed, will cherry pick to branch.
This change gets current file caught up to show working milestone after branch.

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4927

Change-Id: If10e0db2d4acc908f5da7c4e9d30497deef6a2ff
Reviewed-on: https://skia-review.googlesource.com/4927
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Heather Miller <hcm@google.com>
Commit-Queue: Heather Miller <hcm@google.com>