update_engine: Replace NULL with nullptr

Replaced the usage of NULL with nullptr. This also makes it possible to
use standard gtest macros to compare pointers in Update Manager's unit tests.
So, there is no need in custom UMTEST_... macros which are replaced with the
gtest macros (see change in update_engine/update_manager/umtest_utils.h):

UMTEST_ASSERT_NULL(p)      => ASSERT_EQ(nullptr, p)
UMTEST_ASSERT_NOT_NULL(p)  => ASSERT_NE(nullptr, p)
UMTEST_EXPECT_NULL(p)      => EXPECT_EQ(nullptr, p)
UMTEST_EXPECT_NOT_NULL(p)  => EXPECT_NE(nullptr, p)

BUG=None
TEST=FEATURES=test emerge-link update_engine
     USE="clang asan" FEATURES=test emerge-link update_engine

Change-Id: I77a42a1e9ce992bb2f9f263db5cf75fe6110a4ec
Reviewed-on: https://chromium-review.googlesource.com/215136
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Reviewed-by: Alex Deymo <deymo@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>
diff --git a/p2p_manager.h b/p2p_manager.h
index 0e6e1b4..b2cb999 100644
--- a/p2p_manager.h
+++ b/p2p_manager.h
@@ -49,7 +49,7 @@
   typedef base::Callback<void(const std::string& url)> LookupCallback;
 
   // Use the device policy specified by |device_policy|. If this is
-  // NULL, then no device policy is used.
+  // null, then no device policy is used.
   virtual void SetDevicePolicy(const policy::DevicePolicy* device_policy) = 0;
 
   // Returns true if - and only if - P2P should be used on this
@@ -133,7 +133,7 @@
   virtual ssize_t FileGetExpectedSize(const std::string& file_id) = 0;
 
   // Gets whether the file identified by |file_id| is publicly
-  // visible. If |out_result| is not NULL, the result is returned
+  // visible. If |out_result| is not null, the result is returned
   // there. Returns false if an error occurs.
   virtual bool FileGetVisible(const std::string& file_id,
                               bool *out_result) = 0;
@@ -152,7 +152,7 @@
   // Creates a suitable P2PManager instance and initializes the object
   // so it's ready for use. The |file_extension| parameter is used to
   // identify your application, use e.g. "cros_au".  If
-  // |configuration| is non-NULL, the P2PManager will take ownership
+  // |configuration| is non-null, the P2PManager will take ownership
   // of the Configuration object and use it (hence, it must be
   // heap-allocated).
   //