p2p: Use p2p for updates

This is the main patch for enabling use of p2p for consuming and/or
sharing updates via p2p. Refer to the ddoc and other documentation for
how this works.

BUG=chromium:260426,chromium:273110
TEST=New unit tests + unit tests pass + manual testing
Change-Id: I6bc3bddae1e041ccc176969a651396e8e89cb3f0
Reviewed-on: https://chromium-review.googlesource.com/64829
Reviewed-by: David Zeuthen <zeuthen@chromium.org>
Commit-Queue: David Zeuthen <zeuthen@chromium.org>
Tested-by: David Zeuthen <zeuthen@chromium.org>
diff --git a/delta_performer.cc b/delta_performer.cc
index 0a33811..68161d2 100644
--- a/delta_performer.cc
+++ b/delta_performer.cc
@@ -488,6 +488,10 @@
   return true;
 }
 
+bool DeltaPerformer::IsManifestValid() {
+  return manifest_valid_;
+}
+
 bool DeltaPerformer::CanPerformInstallOperation(
     const chromeos_update_engine::DeltaArchiveManifest_InstallOperation&
     operation) {