Merge to XFA: Make DEPS use relative paths to fix recipe

Context: DEPS was originally added to pdfium to allow its recipe to
automatically check out its dependencies. The recipe used absolute
paths, and so the checkout directory's name ('pdfium') had to be
prefixed to the directory checkout of every dependency, as gclient in
the recipe worked one level above the pdfium checkout itself.

After change fe4537269fc7133320a5131638757f2ffa6bd854, the recipe no
longer worked. To fix this, I'm making DEPS use relative paths.

BUG=375773
TBR=tsepez@chromium.org

Review URL: https://codereview.chromium.org/724113003

(cherry picked from commit 39c6b4c50bb06336c2b592a407efa862ff6ed8ae)

Review URL: https://codereview.chromium.org/1421703002 .
diff --git a/DEPS b/DEPS
index f02ac68..ce09f59 100644
--- a/DEPS
+++ b/DEPS
@@ -1,3 +1,5 @@
+use_relative_paths = True
+
 deps = {
   "build/gyp":
     "https://chromium.googlesource.com/external/gyp",