Make GlobalDescriptors::MaybeGet return -1 when the key is not found.

base/global_descriptors_posix.cc

Index: base/global_descriptors_posix.cc
diff --git a/base/global_descriptors_posix.cc b/base/global_descriptors_posix.cc
index d8884a5bae56cf8acc93a2bf30c32db5a3f05fe2..564785df8b9a8b67c0dbdf9df9c75538ea2baa32 100644
--- a/base/global_descriptors_posix.cc
+++ b/base/global_descriptors_posix.cc
@@ -34,9 +34,7 @@ int GlobalDescriptors::MaybeGet(Key key) const {
       return i->second;
   }
 
-  // In order to make unittests pass, we define a default mapping from keys to
-  // descriptors by adding a fixed offset:
-  return kBaseDescriptor + key;
+  return -1;
 }
 
 void GlobalDescriptors::Set(Key key, int fd) {