[PDF] Add unpremultiply support and a GM (try5)

This is a resubmit of https://codereview.chromium.org/22831039 which was
reverted because it causes issues on Windows.

BUG=chromium:175548
R=edisonn@google.com

Review URL: https://codereview.chromium.org/37873003

git-svn-id: http://skia.googlecode.com/svn/trunk@11945 2bbb7eff-a529-9590-31e7-b0007b416f81
3 files changed