XFA: merge patch from CL 828183002, remove static declared SymbolDictCache

Remove static declared SymbolDictCache.

This is a follow up CL on https://codereview.chromium.org/761313004/

BUG=https://code.google.com/p/pdfium/issues/detail?id=93
R=brucedawson@chromium.org, tsepez@chromium.org

Review URL: https://codereview.chromium.org/828183002
4 files changed