Use proper pagination helper

Wrap dependencies into collection so it responds to page
And use Gitlab::Serializer::Pagination to introduce
proper pagination and add all needed headers and params
to response

ee/app/controllers/projects/security/dependencies_controller.rb

@@ -10,8 +10,7 @@ module Projects
           format.json do
             ::Gitlab::UsageCounters::DependencyList.increment(project.id)
 
-            render json: ::DependencyListSerializer.new(project: project)
-                           .represent(paginated_dependencies, build: build)
+            render json: serializer.represent(dependencies, build: build)
           end
         end
       end
@@ -27,17 +26,18 @@ module Projects
                   .last
       end
 
+      def collect_dependencies
+        found_dependencies = build&.success? ? service.execute : []
+        ::Gitlab::DependenciesCollection.new(found_dependencies)
+      end
+
       def ensure_dependency_list_feature_available
         render_404 unless ::Feature.enabled?(:dependency_list) &&
           project.feature_available?(:dependency_list)
       end
 
       def dependencies
-        @dependencies ||= build&.success? ? service.execute : []
-      end
-
-      def paginated_dependencies
-        params[:page] ? Kaminari.paginate_array(dependencies).page(params[:page]) : dependencies
+        @dependencies ||= collect_dependencies
       end
 
       def pipeline
@@ -51,6 +51,12 @@ module Projects
         end
       end
 
+      def serializer
+        serializer = ::DependencyListSerializer.new(project: project)
+        serializer = serializer.with_pagination(request, response) if params[:page]
+        serializer
+      end
+
       def service
         ::Security::DependencyListService.new(pipeline: pipeline, params: query_params)
       end

ee/app/serializers/dependency_list_serializer.rb

 # frozen_string_literal: true
 
 class DependencyListSerializer < BaseSerializer
+  include WithPagination
+
   entity DependencyListEntity
 end

ee/lib/gitlab/dependencies_collection.rb

+# frozen_string_literal: true
+
+module Gitlab
+  class DependenciesCollection
+    include Enumerable
+
+    def initialize(dependencies)
+      @collection = dependencies
+    end
+
+    def each
+      collection.each { |item| yield item }
+    end
+
+    def page(number)
+      Kaminari.paginate_array(collection).page(number)
+    end
+
+    def to_ary
+      collection
+    end
+
+    private
+
+    attr_reader :collection
+  end
+end

ee/spec/controllers/projects/security/dependencies_controller_spec.rb

@@ -83,6 +83,7 @@ describe Projects::Security::DependenciesController do
 
               it 'returns paginated list' do
                 expect(json_response['dependencies'].length).to eq(1)
+                expect(response).to include_pagination_headers
               end
             end
           end

ee/spec/lib/gitlab/dependencies_collection_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Gitlab::DependenciesCollection do
+  let(:collection) { described_class.new(fake_dependencies) }
+  let(:fake_dependencies) { Array.new(42, :dependency) }
+
+  it 'responds to each' do
+    expect(collection).to respond_to(:each)
+  end
+
+  describe '#page' do
+    subject { collection.page(3) }
+
+    it 'returns paginated collection' do
+      expect(subject.length).to eq(2)
+    end
+  end
+
+  describe '#to_ary' do
+    subject { collection.to_ary }
+
+    it 'returns Array' do
+      is_expected.to be_an(Array)
+    end
+  end
+end