Manually roll the engine to the last good commit known to the
auto-roller.

git log 0c2b6dc6e339..f6af1f20ba92 --no-merges --oneline
f6af1f20b Roll src/third_party/skia e8e2bb1384ad..3b8b11e1f912 (4 commits) (flutter/engine#6395)
7f2e2ea4f Roll src/third_party/skia bba2921c1970..e8e2bb1384ad (1 commits) (flutter/engine#6394)
4f69beb27 Roll src/third_party/skia 7f3ceba5dc7f..bba2921c1970 (1 commits) (flutter/engine#6392)
a4a5449d5 Roll src/third_party/skia 22867ce6fe83..7f3ceba5dc7f (1 commits) (flutter/engine#6391)
31278f0b8 Roll src/third_party/skia ebfbf1d749c1..22867ce6fe83 (1 commits) (flutter/engine#6390)
73409c228 Roll src/third_party/skia 93d71666b2f1..ebfbf1d749c1 (1 commits) (flutter/engine#6389)
6f8c827b2 Dart SDK roll for 2018-09-28 (flutter/engine#6388)
6e96ae542 Roll src/third_party/skia cfa315390f04..93d71666b2f1 (6 commits) (flutter/engine#6386)
39dee5b61 Fix Fuchsia build. (flutter/engine#6383)
560eba1a3 Roll src/third_party/skia 87311c6e565f..cfa315390f04 (9 commits) (flutter/engine#6382)