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/omaha_request_action.cc b/omaha_request_action.cc
index 1bd5041..ede4e64 100644
--- a/omaha_request_action.cc
+++ b/omaha_request_action.cc
@@ -101,7 +101,7 @@
                   int ping_roll_call_days,
                   PrefsInterface* prefs) {
   string app_body;
-  if (event == NULL) {
+  if (event == nullptr) {
     app_body = GetPingXml(ping_active_days, ping_roll_call_days);
     if (!ping_only) {
       // not passing update_disabled to Omaha because we want to
@@ -428,7 +428,7 @@
 // static
 int OmahaRequestAction::GetInstallDate(SystemState* system_state) {
   PrefsInterface* prefs = system_state->prefs();
-  if (prefs == NULL)
+  if (prefs == nullptr)
     return -1;
 
   // If we have the value stored on disk, just return it.
@@ -935,7 +935,7 @@
   int64_t manifest_metadata_size = 0;
   int64_t next_data_offset = 0;
   int64_t next_data_length = 0;
-  if (system_state_ != NULL &&
+  if (system_state_ &&
       system_state_->prefs()->GetInt64(kPrefsManifestMetadataSize,
                                        &manifest_metadata_size) &&
       manifest_metadata_size != -1 &&
@@ -948,7 +948,7 @@
   }
 
   string file_id = utils::CalculateP2PFileId(response.hash, response.size);
-  if (system_state_->p2p_manager() != NULL) {
+  if (system_state_->p2p_manager()) {
     LOG(INFO) << "Checking if payload is available via p2p, file_id="
               << file_id << " minimum_size=" << minimum_size;
     system_state_->p2p_manager()->LookupUrlForFile(
@@ -1190,7 +1190,7 @@
 // static
 bool OmahaRequestAction::HasInstallDate(SystemState *system_state) {
   PrefsInterface* prefs = system_state->prefs();
-  if (prefs == NULL)
+  if (prefs == nullptr)
     return false;
 
   return prefs->Exists(kPrefsInstallDateDays);
@@ -1204,7 +1204,7 @@
   TEST_AND_RETURN_FALSE(install_date_days >= 0);
 
   PrefsInterface* prefs = system_state->prefs();
-  if (prefs == NULL)
+  if (prefs == nullptr)
     return false;
 
   if (!prefs->SetInt64(kPrefsInstallDateDays, install_date_days))