Move zlib to a DEPS-based checkout.

BUG=chromium:541704

Change-Id: I313d8853450ff5940b3baa4054ff441b342c46b7
Reviewed-on: https://pdfium-review.googlesource.com/3350
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: dsinclair <dsinclair@chromium.org>
Reviewed-by: Nicolás Peña <npm@chromium.org>
diff --git a/.gitignore b/.gitignore
index 5a4dcc7..c80bf34 100644
--- a/.gitignore
+++ b/.gitignore
@@ -13,6 +13,7 @@
 /third_party/llvm
 /third_party/llvm-build
 /third_party/skia
+/third_party/zlib
 /tools/clang
 /tools/generate_library_loader
 /tools/gyp