Merge to XFA: Fix some -Wmaybe-uninitialized errors.

- Break up CPDF_DIBSource::DownSampleScanline() into smaller functions.

R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/1287843004 .

(cherry picked from commit 9ae9ae2a68439cbc84a719c0dacf6984703e77cb)

Review URL: https://codereview.chromium.org/1304873002 .
4 files changed