Leak per-isolate data in pdfium

Right now we're freeing per-isolate data everytime a document is destroyed even
though it may be in use by other documents. For now we leak the per-isolate
data until crbug.com/531339 is fixed.

BUG=531339
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/1372353002 .

Patch from Raymes Khoury <raymes@chromium.org>.