Fix 'gn gen' with target_os="win" on Linux.

This is the first step to enable Windows cross-compilation when the
host_os is Linux.

There are compilation errors ahead but at least it is possible to
generate ninja files.

Bug: webrtc:8875
Change-Id: I91a238bcb5e8f7670a6d19805e1ac032511fd46e
Reviewed-on: https://webrtc-review.googlesource.com/51821
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21984}
diff --git a/DEPS b/DEPS
index 0fddf9f..d23873d 100644
--- a/DEPS
+++ b/DEPS
@@ -232,11 +232,22 @@
                '--arch=amd64'],
   },
   {
+    # Case-insensitivity for the Win SDK. Must run before win_toolchain below.
+    'name': 'ciopfs_linux',
+    'pattern': '.',
+    'condition': 'checkout_win and host_os == "linux"',
+    'action': [ 'python',
+                'src/third_party/depot_tools/download_from_google_storage.py',
+                '--no_resume',
+                '--no_auth',
+                '--bucket', 'chromium-browser-clang/ciopfs',
+                '-s', 'src/build/ciopfs.sha1',
+    ]
+  },
+  {
     # Update the Windows toolchain if necessary. Must run before 'clang' below.
     'name': 'win_toolchain',
     'pattern': '.',
-    # TODO(thakis): Put some condition here. Not just host_os == 'win', because
-    # we also need this for (mac|linux) -> win cross builds.
     'action': ['python', 'src/build/vs_toolchain.py', 'update'],
   },
   {