Improve PNG decode performance: avoid branching in the row write loop

No change in behavior, no new tests.

BUG=258324

Review URL: https://codereview.chromium.org/18099004

git-svn-id: svn://svn.chromium.org/blink/trunk@175666 bbb929c8-8fbe-4397-9dbb-9b2b20218538