Remove unneeded flags from NativeImageSkia::drawPattern()

Minor cleanup: some suggestions didn't make it in the final version of
https://codereview.chromium.org/412353003/

R=schenney@chromium.org

Review URL: https://codereview.chromium.org/428573003

git-svn-id: svn://svn.chromium.org/blink/trunk@179039 bbb929c8-8fbe-4397-9dbb-9b2b20218538