[build] Port: Fetch gn based on the host OS.

This ports:
https://crrev.com/c/1526276

TBR=tmrts@chromium.org

Bug: chromium:855791
Change-Id: I1370b4f72a1aa22d3ec1c5c7ca4ff07fad306501
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1528999Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Tamer Tas <tmrts@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60315}

DEPS

@@ -38,7 +38,7 @@ deps = {
       }
     ],
     'dep_type': 'cipd',
-    'condition': 'checkout_linux',
+    'condition': 'host_os == "linux"',
   },
   'v8/buildtools/mac': {
     'packages': [
@@ -48,7 +48,7 @@ deps = {
       }
     ],
     'dep_type': 'cipd',
-    'condition': 'checkout_mac',
+    'condition': 'host_os == "mac"',
   },
   'v8/buildtools/third_party/libc++/trunk':
     Var('chromium_url') + '/chromium/llvm-project/libcxx.git' + '@' + 'a50f5035629b7621e92acef968403f71b7d48553',
@@ -64,7 +64,7 @@ deps = {
       }
     ],
     'dep_type': 'cipd',
-    'condition': 'checkout_win',
+    'condition': 'host_os == "win"',
   },
   'v8/base/trace_event/common':
     Var('chromium_url') + '/chromium/src/base/trace_event/common.git' + '@' + '936ba8a963284a6b3737cf2f0474a7131073abee',