[build] Add common directory for custom deps

NOTRY=true

Bug: chromium:846711
Change-Id: I5d7f366db2e80fbe539b34a5dcb7475e4e38bd29
Reviewed-on: https://chromium-review.googlesource.com/1088694Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53621}

custom_deps/.gitignore

+*
+!.gitignore
+!OWNERS
+!README.md
\ No newline at end of file

custom_deps/OWNERS

+machenbach@chromium.org
+sergiyb@chromium.org
\ No newline at end of file

custom_deps/README.md

+Common directory for custom dependencies pulled in via .gclient custom_deps.
+All subdirectories are ignored by git by default.
\ No newline at end of file