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/update_manager/evaluation_context.cc b/update_manager/evaluation_context.cc
index 5bee226..e334560 100644
--- a/update_manager/evaluation_context.cc
+++ b/update_manager/evaluation_context.cc
@@ -146,7 +146,7 @@
 
 bool EvaluationContext::RunOnValueChangeOrTimeout(Closure callback) {
   // Check that the method was not called more than once.
-  if (callback_.get() != nullptr) {
+  if (callback_.get()) {
     LOG(ERROR) << "RunOnValueChangeOrTimeout called more than once.";
     return false;
   }