[chrome/m77] [infra] Upgrade MacBookAir to 10.15.

Also add a NativeFonts GPU bot pending the availability of CPU bots
(https://bugs.chromium.org/p/chromium/issues/detail?id=1018242).

Change-Id: I16c19ae2e05be16b622c14289e255d0a342c0654
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/251817
Reviewed-by: Kevin Lubick <kjlubick@google.com>
(cherry picked from commit 5b6ccbb6b59974d80c095958cf3fd77fac194caa)
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/252195
Commit-Queue: Ben Wagner aka dogben <benjaminwagner@google.com>