CompositorFrameReportingController: Do not rely on C++20 features in DCHECK_NE call

Follow-up to commit a572c15d ("Report the latency increase of pipeline
stages in missed frames compared to not missed frames"), which ended up
reverting commit aeed4d1f ("libstdc++: do not assume unique_ptr has ostream
operator"). A call that looks like

    DCHECK_NE(foo, nullptr);

where |foo| is a std::unique_ptr fails with libstdc++ because there is no
operator<< overload for std::unique_ptr until C++20 (libc++ just happens to
have it):

../../cc/scheduler/compositor_frame_reporting_controller.cc:60:3: error: no matching function for call to 'MakeCheckOpValueString'
  DCHECK_NE(reporters_[PipelineStage::kBeginMainFrame],
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../base/logging.h:851:31: note: expanded from macro 'DCHECK_NE'
                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../base/logging.h:822:29: note: expanded from macro 'DCHECK_OP'
  EAT_STREAM_PARAMETERS << (::logging::MakeCheckOpValueString(      \
                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../base/logging.h:635:18: note: candidate function not viable: no known conversion from 'std::unique_ptr<CompositorFrameReporter>' to 'std::nullptr_t'
      (aka 'nullptr_t') for 2nd argument
BASE_EXPORT void MakeCheckOpValueString(std::ostream* os, std::nullptr_t p);
                 ^
../../base/logging.h:596:1: note: candidate template ignored: requirement 'base::internal::SupportsOstreamOperator<const unique_ptr<CompositorFrameReporter,
      default_delete<CompositorFrameReporter> > &>::value' was not satisfied [with T = std::unique_ptr<cc::CompositorFrameReporter,
      std::default_delete<cc::CompositorFrameReporter> >]
MakeCheckOpValueString(std::ostream* os, const T& v) {
^
../../base/logging.h:606:1: note: candidate template ignored: requirement 'base::internal::SupportsToString<const unique_ptr<CompositorFrameReporter,
      default_delete<CompositorFrameReporter> > &>::value' was not satisfied [with T = std::unique_ptr<cc::CompositorFrameReporter,
      std::default_delete<cc::CompositorFrameReporter> >]
MakeCheckOpValueString(std::ostream* os, const T& v) {
^
../../base/logging.h:619:1: note: candidate template ignored: requirement 'std::is_function<typename std::remove_pointer<unique_ptr<CompositorFrameReporter,
      default_delete<CompositorFrameReporter> > >::type>::value' was not satisfied [with T = std::unique_ptr<cc::CompositorFrameReporter,
      std::default_delete<cc::CompositorFrameReporter> >]
MakeCheckOpValueString(std::ostream* os, const T& v) {
^
../../base/logging.h:630:1: note: candidate template ignored: requirement 'std::is_enum<unique_ptr<CompositorFrameReporter,
      default_delete<CompositorFrameReporter> > >::value' was not satisfied [with T = std::unique_ptr<cc::CompositorFrameReporter,
      std::default_delete<cc::CompositorFrameReporter> >]
MakeCheckOpValueString(std::ostream* os, const T& v) {
^

Bug: 957519
Change-Id: Icbc23eb23b4641a64dfeb00d27ea1d393dda232a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1672860
Commit-Queue: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Auto-Submit: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Reviewed-by: Sunny Sachanandani <sunnyps@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672024}

cc/scheduler/compositor_frame_reporting_controller.cc

@@ -57,8 +57,10 @@ void CompositorFrameReportingController::WillBeginImplFrame() {
 
 void CompositorFrameReportingController::WillBeginMainFrame() {
   DCHECK(reporters_[PipelineStage::kBeginImplFrame]);
-  DCHECK_NE(reporters_[PipelineStage::kBeginMainFrame],
-            reporters_[PipelineStage::kBeginImplFrame]);
+  // We need to use .get() below because operator<< in std::unique_ptr is a
+  // C++20 feature.
+  DCHECK_NE(reporters_[PipelineStage::kBeginMainFrame].get(),
+            reporters_[PipelineStage::kBeginImplFrame].get());
   reporters_[PipelineStage::kBeginImplFrame]->StartStage(
       CompositorFrameReporter::StageType::kSendBeginMainFrameToCommit, Now(),
       stage_history_[static_cast<int>(CompositorFrameReporter::StageType::