Make checkdeps.py check third_party.

For this first cl, it's only checking checked in code. I've had to add DEPS files to make it pass, since some of this code legitimately uses base etc...

BUG=469692
R=brettw@chromium.org

Review URL: https://codereview.chromium.org/1045863002

Cr-Original-Commit-Position: refs/heads/master@{#323302}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 5b6373fbaed471c766ec691ad2c816abbc482d29
diff --git a/jsoncpp.gypi b/jsoncpp.gypi
index 3ce61e1..dfad805 100644
--- a/jsoncpp.gypi
+++ b/jsoncpp.gypi
@@ -24,6 +24,7 @@
       'source/src/lib_json/json_writer.cpp',
     ],
     'include_dirs': [
+      '../..',
       'overrides/include/',
       'source/include/',
       'source/src/lib_json/',
diff --git a/overrides/include/json/value.h b/overrides/include/json/value.h
index a7d7c07..1484ee2 100644
--- a/overrides/include/json/value.h
+++ b/overrides/include/json/value.h
@@ -7,7 +7,7 @@
 # define CPPTL_JSON_H_INCLUDED
 
 #if !defined(JSON_IS_AMALGAMATION)
-# include "json/forwards.h"
+# include "third_party/jsoncpp/source/include/json/forwards.h"
 #endif // if !defined(JSON_IS_AMALGAMATION)
 # include <string>
 # include <vector>