Update DEPS to always pull clang.

If the --if-needed argument is passed to the clang update script then we depend
on GYP_DEFINES to be set in order to update clang. We no longer define
GYP_DEFINES so we fail to update clang (or checkout clang).

BUG=chromium:647380

Review-Url: https://codereview.chromium.org/2350093002
diff --git a/DEPS b/DEPS
index e57e5af..70d5af9 100644
--- a/DEPS
+++ b/DEPS
@@ -162,12 +162,11 @@
     ],
   },
   {
-    # Pull clang if needed or requested via GYP_DEFINES.
+    # Pull clang
     'name': 'clang',
     'pattern': '.',
     'action': ['python',
-               'pdfium/tools/clang/scripts/update.py',
-               '--if-needed'
+               'pdfium/tools/clang/scripts/update.py'
     ],
   },
   {