Make libunwind DEPS entry Android-only

Other platforms do not need this library. Slightly reduce the size of
the checkout for those not building on Android.

Change-Id: I678120b4c59dbb63b7e45cbe2ed61127310bf65d
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/129331
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Thomas Sepez <tsepez@google.com>

DEPS

diff --git a/DEPS b/DEPS
index d6923f6..0376ad2 100644
--- a/DEPS
+++ b/DEPS
@@ -392,10 +392,12 @@
         '/external/github.com/llvm/llvm-project/libcxxabi.git@' +
         Var('libcxxabi_revision'),
 
-  'third_party/libunwind/src':
-    Var('chromium_git') +
+  'third_party/libunwind/src': {
+    'url': Var('chromium_git') +
         '/external/github.com/llvm/llvm-project/libunwind.git@' +
         Var('libunwind_revision'),
+    'condition': 'checkout_android',
+  },
 
   'third_party/libjpeg_turbo':
     Var('chromium_git') + '/chromium/deps/libjpeg_turbo.git@' +