Add dpranke@chromium.org to //third_party/OWNERS.

In many cases, we need to change //third_party files due to
build changes or restructuring dependencies, and requiring an
ENG_REVIEW_OWNER for that is overkill, but  having a backup to
thakis@ is probably a good idea.

Change-Id: Icdba6251fcefe216c2dc97be911615d32b934427
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1879811Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Darin Fisher <darin@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709366}

third_party/OWNERS

@@ -20,6 +20,11 @@ chromium-third-party@google.com
 
 thakis@chromium.org
 
+# For build changes and structural changes to the repos (i.e.,
+# DEPS-related changes). For adding new third-party dependencies
+# please use one of the other reviewers.
+dpranke@chromium.org
+
 per-file .gitignore=*
 
 # Translation artifacts: