Re-enable pdfium_unittests.

In order for the standalone pdfium build to succeed,
DEPS is updated to pull in gtest, and a stub gtest.gyp
file is provided (which is unused under a chromium
checkout).

R=brucedawson@chromium.org, thakis@chromium.org

Review URL: https://codereview.chromium.org/835233002
4 files changed