add flag to stage future skia CL https://codereview.chromium.org/1533953002/
add flag to stage future skia CL https://codereview.chromium.org/1533953002/

BUG=
NOTRY=True
just add a flag to allow skia DEPS roll

Review URL: https://codereview.chromium.org/1535483003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#365872}

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