Use enclosing int rect for picture shader tiles

To avoid rounding to zero for very small tiles.

BUG=chromium:590447
R=reed@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1749763002

Review URL: https://codereview.chromium.org/1749763002
1 file changed