Make checking out testing/corpus optional.

In the root DEPS file, add a checkout_testing_corpus variable for
controlling whether testing/corpus gets checked out or not.

Change-Id: Ie7a19fcfac4352a5d44bff9703db9f1f842f29dc
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/95272
Reviewed-by: Nigi <nigi@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/DEPS b/DEPS
index a2b8e9d..5bee48e 100644
--- a/DEPS
+++ b/DEPS
@@ -7,9 +7,10 @@
 
 vars = {
   # By default, we should check out everything needed to run on the main
-  # chromium waterfalls. This var can be also be set to 'small', in order
-  # to skip things are not strictly needed to build chromium for development
-  # purposes.
+  # pdfium waterfalls. This var can be also be set to 'small', in order to skip
+  # things are not strictly needed to build pdfium for development purposes,
+  # by adding the following line to the .gclient file inside a solutions entry:
+  #      "custom_vars": { "checkout_configuration": "small" },
   'checkout_configuration': 'default',
 
   # By default, do not check out Google Benchmark. This only exists to satisfy
@@ -19,6 +20,8 @@
 
   'checkout_instrumented_libraries': 'checkout_linux and checkout_configuration != "small"',
 
+  'checkout_testing_corpus': 'checkout_configuration != "small"',
+
   # By default, download the fuchsia sdk from the public sdk directory.
   'fuchsia_sdk_cipd_prefix': 'fuchsia/sdk/gn/',
 
@@ -220,8 +223,10 @@
     'condition': 'host_os == "win"',
   },
 
-  'testing/corpus':
-    Var('pdfium_git') + '/pdfium_tests@' + Var('pdfium_tests_revision'),
+  'testing/corpus': {
+    'url': Var('pdfium_git') + '/pdfium_tests@' + Var('pdfium_tests_revision'),
+    'condition': 'checkout_testing_corpus',
+  },
 
   'third_party/abseil-cpp':
     Var('chromium_git') + '/chromium/src/third_party/abseil-cpp.git@' +