Merge to XFA: Pull our new test case repository via deps.

Original Review URL: https://codereview.chromium.org/1010833003
TBR=thestig@chromium.org

Review URL: https://codereview.chromium.org/1010973002
diff --git a/DEPS b/DEPS
index a7170c8..6ad123e 100644
--- a/DEPS
+++ b/DEPS
@@ -2,6 +2,9 @@
   "build/gyp":
     "https://chromium.googlesource.com/external/gyp",
 
+  "testing/corpus":
+     "https://pdfium.googlesource.com/pdfium_tests",
+
   "testing/gmock":
      "https://chromium.googlesource.com/external/googlemock.git@29763965ab52f24565299976b936d1265cb6a271",