Pull in specific revision of PDF corpus via DEPS.

There are two reasons for doing this. The first is that adding files
to this repository doesn't trigger a rebuild on the bot, so the next
unrelated pdfium change will be the one to hit any issues with the
unversioned corpus.

The second is that we may need a specific version of the expected output
files (.pngs) to go with a specific pdfium build.

R=thestig@chromium.org

Review URL: https://codereview.chromium.org/1019783003
diff --git a/DEPS b/DEPS
index d70e756..6df5c3a 100644
--- a/DEPS
+++ b/DEPS
@@ -5,7 +5,7 @@
     "https://chromium.googlesource.com/external/gyp",
 
   "testing/corpus":
-     "https://pdfium.googlesource.com/pdfium_tests",
+     "https://pdfium.googlesource.com/pdfium_tests@1b25cea6e0472d89df747c9814ee4cb028c51247",
 
   "testing/gmock":
      "https://chromium.googlesource.com/external/googlemock.git@29763965ab52f24565299976b936d1265cb6a271",