chrome: specify locking mode in calls to base::File::Lock().

The distinction between kShared and kExplicit is large enough that there
are significant readability advantages to having the default be explicit
in the code.

Bug: 1125315
Change-Id: Ifeeedc4f51e44e5348177c67d48361da1ac79cc0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2394720
Auto-Submit: Victor Costan <pwnall@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Commit-Queue: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804814}

chrome/credential_provider/gaiacp/user_policies_manager.cc

@@ -83,7 +83,8 @@ std::unique_ptr<base::File> GetOpenedPolicyFileForUser(
     return nullptr;
   }
 
-  base::File::Error lock_error = policy_file->Lock();
+  base::File::Error lock_error =
+      policy_file->Lock(base::File::LockMode::kExclusive);
   if (lock_error != base::File::FILE_OK) {
     LOGFN(ERROR) << "Failed to obtain exclusive lock on policy file! Error: "
                  << lock_error;