Disable JBIG2 cache; prevent data corruption - try 2.

Also change CJBig2_SymbolDict::DeepCopy() to return a unique_ptr to
prevent a potential leak if the cache size was 0.

BUG=pdfium:207
R=tsepez@chromium.org, jbreiden@google.com

Review URL: https://codereview.chromium.org/1374633004 .
3 files changed