[PDF] Fixes the cases where SkPDFDevice::finishContentEntry is called with empty...

[PDF] Fixes the cases where SkPDFDevice::finishContentEntry is called with empty content.

There are some cases (like drawing a completely transparent image) where the code tries to finish a content entry with no content and an xfermode other than clear.  This makes those cases work correctly.

This is likely what was happening in chromium:316546, but it wasn't clear what the core problem was. desk_techcrunch.skp tripped a debug only assert that exposed the core issue.

BUG=skia:1868,chromium:316546
R=reed@google.com, bungeman@google.com

Author: vandebo@chromium.org

Review URL: https://codereview.chromium.org/92453002

git-svn-id: http://skia.googlecode.com/svn/trunk@12473 2bbb7eff-a529-9590-31e7-b0007b416f81