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/mock_http_fetcher.h b/mock_http_fetcher.h
index 748acf1..4bf1685 100644
--- a/mock_http_fetcher.h
+++ b/mock_http_fetcher.h
@@ -36,7 +36,7 @@
                   ProxyResolver* proxy_resolver)
       : HttpFetcher(proxy_resolver, &fake_system_state_),
         sent_size_(0),
-        timeout_source_(NULL),
+        timeout_source_(nullptr),
         timout_tag_(0),
         paused_(false),
         fail_transfer_(false),
@@ -120,7 +120,7 @@
   // time out for 0s just to make sure that run loop services other clients.
   GSource* timeout_source_;
 
-  // ID of the timeout source, valid only if timeout_source_ != NULL
+  // ID of the timeout source, valid only if timeout_source_ != null
   guint timout_tag_;
 
   // True iff the fetcher is paused.