Use ToUnicode mapping even when unicode is 0.

CPDF_Font::UnicodeFromCharcode returns 0 only if ToUnicode map maps the
charcode to 0. CPDF_SimpleFont::UnicodeFromCharcode and CPDF_CID_Font::
UnicodeFromCharCode return 0 only if the call to CPDF_Font returns 0.
In other cases, these methods return an empty string. So when
processing text, a 0 return from the method should not be replaced
with the charcode.

BUG=pdfium:583

Review-Url: https://codereview.chromium.org/2342073002

1 files changed, 216 insertions, 0 deletions

diff --git a/testing/resources/bug_583.pdf b/testing/resources/bug_583.pdf
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