Add SK_SUPPORT_LEGACY_PICTURESHADER_ROUNDING Skia option
Add SK_SUPPORT_LEGACY_PICTURESHADER_ROUNDING Skia option

Guard for upcoming Skia change.

BUG=chromium:590447
TBR=reed@google.com
NOTRY=true

Review URL: https://codereview.chromium.org/1747963002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#378225}

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