p2p: Make P2PManager available from the SystemState singleton

BUG=chromium:260426
TEST=Unit tests pass
Change-Id: Iadaa531d631c8d7268f35d512ff33f8d53e1cc5e
Reviewed-on: https://chromium-review.googlesource.com/64828
Reviewed-by: David Zeuthen <zeuthen@chromium.org>
Commit-Queue: David Zeuthen <zeuthen@chromium.org>
Tested-by: David Zeuthen <zeuthen@chromium.org>
diff --git a/constants.h b/constants.h
index ce80389..e114956 100644
--- a/constants.h
+++ b/constants.h
@@ -99,6 +99,9 @@
 // two days.
 const int kMaxP2PAttemptTimeSeconds = 2*24*60*60;
 
+// The maximum number of payload files to keep in /var/cache/p2p.
+const int kMaxP2PFilesToKeep = 3;
+
 // The default number of UMA buckets for metrics.
 const int kNumDefaultUmaBuckets = 50;