Add an allowed_hosts entry in DEPS.

Provide a sanity check to make sure DEPS does not actually pull in
random git repos.

Change-Id: I1be5283cea4255a1ac7d63d8f16217a940d95cad
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/87493
Reviewed-by: Hui Yingst <nigi@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>

DEPS

diff --git a/DEPS b/DEPS
index 88becc8..c6fcbf9 100644
--- a/DEPS
+++ b/DEPS
@@ -143,6 +143,15 @@
   'zlib_revision': '2231b7c84b443385f2bd65dd6c46b3c7b957eae1',
 }
 
+# Only these hosts are allowed for dependencies in this DEPS file.
+# If you need to add a new host, and the new host is not in Chromium's DEPS
+# file's allowed_hosts list, contact Chrome infrastructure team.
+allowed_hosts = [
+  'chromium.googlesource.com',
+  'pdfium.googlesource.com',
+  'skia.googlesource.com',
+]
+
 deps = {
   'base/trace_event/common':
     Var('chromium_git') + '/chromium/src/base/trace_event/common.git@' +