Enable the page_set of benchmark test file to use actual page set object.

tools/perf/benchmarks/memory.py

 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 from telemetry import test
 
 from measurements import memory
+from page_sets import mobile_memory
+from page_sets import top_25
+from page_sets import top_desktop_sites_2012Q3
+from page_sets import tough_dom_memory_cases
 
 
 @test.Disabled('android')  # crbug.com/370977
 class MemoryMobile(test.Test):
   test = memory.Memory
-  page_set = 'page_sets/mobile_memory.py'
+  page_set = mobile_memory.MobileMemoryPageSet()
 
 
 class MemoryTop25(test.Test):
   test = memory.Memory
-  page_set = 'page_sets/top_25.py'
+  page_set = top_25.Top25PageSet()
 
 
 class Reload2012Q3(test.Test):
   tag = 'reload'
   test = memory.Memory
-  page_set = 'page_sets/top_desktop_sites_2012Q3.py'
+  page_set = top_desktop_sites_2012Q3.Top2012Q3PageSet()
 
 
 @test.Disabled('android')  # crbug.com/371153
 class MemoryToughDomMemoryCases(test.Test):
   test = memory.Memory
-  page_set = 'page_sets/tough_dom_memory_cases.py'
+  page_set = tough_dom_memory_cases.ToughDomMemoryCasesPageSet()