Pull our new test case repository via deps.

Pull our new test case repository via deps.

Add a run_corpus_tests.py script to run pdfium_test against
the corpus tree.

Note that this differs from the run_pixel_tests.py script,
since pre-processing is not required.  I'll work on unifying
these in a subsequent CL.

R=thestig@chromium.org

Committed: https://pdfium.googlesource.com/pdfium/+/b29e12ac58aea7ca43d726e152770e7ea80ab54a

[Tom Sepez, 2015-03-16 18:45:11]

tsepez@chromium.org changed reviewers:
+ jam@chromium.org

[Tom Sepez, 2015-03-16 18:47:58]

John, please review.

As far as naming is concerned, having 

pdfium
pdfium_tests

as the names of the top-level repo's seemed consistent, but having a
pdfium_tests directory in addition to (or under) testing/ didn't add any
clarifying information, and seemed confusing, hence the testcases path.

[Tom Sepez, 2015-03-16 21:09:28]

On 2015/03/16 18:47:58, Tom Sepez wrote:
> hence the testcases path.
s/testcases/corpus/.  The collected pdfs wind up in testing/corpus now.

[Tom Sepez, 2015-03-16 22:02:23]

tsepez@chromium.org changed reviewers:
+ thestig@chromium.org

[Tom Sepez, 2015-03-16 22:02:33]

+thestig@

[Lei Zhang, 2015-03-16 22:20:37]

lgtm

[Tom Sepez, 2015-03-16 22:22:02]

Committed patchset #4 (id:60001) manually as
b29e12ac58aea7ca43d726e152770e7ea80ab54a (presubmit successful).