update_engine: Update libchrome APIS to r456626.

The new libchrome has been ported from Chromium and some APIs
have changed. Make necessary changes at call sites.

BUG=chromium:724678
CQ-DEPEND=CL:480928
Test: Build.

Change-Id: I4dbaea4a2a19031375a8bf2415645a4f226dab57
diff --git a/fake_p2p_manager_configuration.h b/fake_p2p_manager_configuration.h
index 1bc1dc8..c1cf4f2 100644
--- a/fake_p2p_manager_configuration.h
+++ b/fake_p2p_manager_configuration.h
@@ -37,9 +37,7 @@
   }
 
   // P2PManager::Configuration override
-  base::FilePath GetP2PDir() override {
-    return p2p_dir_.path();
-  }
+  base::FilePath GetP2PDir() override { return p2p_dir_.GetPath(); }
 
   // P2PManager::Configuration override
   std::vector<std::string> GetInitctlArgs(bool is_start) override {