Cleanup of component/metrics for GN.

- Adds missing rules of BUILD.gn
- removes unnecessary const which causes warning for some compiler
  flag set on GN.

BUG=None
R=mpearson@chromium.org, cmasone@chromium.org
TEST=manually

Review URL: https://codereview.chromium.org/644603002

Cr-Commit-Position: refs/heads/master@{#299012}

components/metrics/BUILD.gn

@@ -52,6 +52,10 @@ source_set("metrics") {
     "//components/variations",
     "//third_party/zlib",
   ]
+
+  if (is_chromeos) {
+    deps += [ ":serialization" ]
+  }
 }
 
 # GYP version: components/metrics.gypi:metrics_gpu
@@ -121,4 +125,17 @@ static_library("test_support") {
   ]
 }
 
+if (is_linux) {
+  # GYP version: components/metrics.gypi:metrics_serialization
+  source_set("serialization") {
+    sources = [
+      "serialization/serialization_utils.cc",
+      "serialization/serialization_utils.h",
+      "serialization/metric_sample.cc",
+      "serialization/metric_sample.h",
+    ]
+    deps = [ "//base" ]
+  }
+}
+
 # TODO(GYP): metrics_chromeos

components/metrics/serialization/metric_sample.cc

@@ -74,25 +74,25 @@ std::string MetricSample::ToString() const {
   }
 }
 
-const int MetricSample::sample() const {
+int MetricSample::sample() const {
   CHECK_NE(type_, USER_ACTION);
   CHECK_NE(type_, CRASH);
   return sample_;
 }
 
-const int MetricSample::min() const {
+int MetricSample::min() const {
   CHECK_EQ(type_, HISTOGRAM);
   return min_;
 }
 
-const int MetricSample::max() const {
+int MetricSample::max() const {
   CHECK_NE(type_, CRASH);
   CHECK_NE(type_, USER_ACTION);
   CHECK_NE(type_, SPARSE_HISTOGRAM);
   return max_;
 }
 
-const int MetricSample::bucket_count() const {
+int MetricSample::bucket_count() const {
   CHECK_EQ(type_, HISTOGRAM);
   return bucket_count_;
 }

components/metrics/serialization/metric_sample.h

@@ -43,10 +43,10 @@ class MetricSample {
   // Check the metric type to make sure the request make sense. (ex: a crash
   // sample does not have a bucket_count so we crash if we call bucket_count()
   // on it.)
-  const int sample() const;
-  const int min() const;
-  const int max() const;
-  const int bucket_count() const;
+  int sample() const;
+  int min() const;
+  int max() const;
+  int bucket_count() const;
 
   // Returns a serialized version of the sample.
   //