Create pdfium_unittests binary.

PDFium has not yet had a unit-tests binary, so introduce one based on
the typical gtest framework. Also provide a small initial test fragment
for fxcrt strings for instructional purposes.

Naturally, doing so kicked out one corner case that isn't handled, we'll
fix that in a separate patch.

This is a small part of the testing strategy tracking bug.
BUG=https://code.google.com/p/pdfium/issues/detail?id=62
R=brucedawson@chromium.org, palmer@chromium.org

Review URL: https://codereview.chromium.org/831653002
3 files changed