Convert DCHECKs to CHECKs for histogram types

When a histogram is looked up by name, there is currently a DCHECK that
verifies the type of the stored histogram matches the expected type.

A mismatch represents a significant problem because the returned
HistogramBase is cast to a Histogram in ValidateRangeChecksum,
potentially causing a crash.

This CL converts the DCHECK to a CHECK to prevent the possibility of
type confusion in release builds.

BUG=651443
R=isherman@chromium.org

Review-Url: https://codereview.chromium.org/2381893003
Cr-Commit-Position: refs/heads/master@{#421929}
std::unique_ptr<HistogramSamples> Histogram::SnapshotFinalDelta() const {
  DCHECK(!final_delta_created_);
  final_delta_created_ = true;

  std::unique_ptr<HistogramSamples> snapshot = SnapshotSampleVector();

  if (logged_samples_)
    snapshot->Subtract(*logged_samples_);
  return snapshot;
}

std::unique_ptr<HistogramSamples> Histogram::SnapshotFinalDelta() const {
  DCHECK(!final_delta_created_);
  final_delta_created_ = true;

  std::unique_ptr<HistogramSamples> snapshot = SnapshotSampleVector();

  if (logged_samples_)
    snapshot->Subtract(*logged_samples_);
  return snapshot;
}
