[telemetry] Add new measurement that counts number of GCs needed to free V8 context.

tools/perf/benchmarks/v8.py

 # Copyright 2014 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.
 
 import page_sets
+from measurements import v8_detached_context_age_in_gc
 from measurements import v8_gc_times
 from telemetry import benchmark
 
 
 @benchmark.Disabled('win')  # crbug.com/416502
 class V8GarbageCollectionCases(benchmark.Benchmark):
   """Measure V8 GC metrics on the garbage collection cases."""
   test = v8_gc_times.V8GCTimes
   page_set = page_sets.GarbageCollectionCasesPageSet
 
@@ skipping 19 matching lines @@
 class V8KeyMobileSites(benchmark.Benchmark):
   """Measures V8 GC metrics on the while scrolling down key mobile sites.
 
   http://www.chromium.org/developers/design-documents/rendering-benchmarks"""
   test = v8_gc_times.V8GCTimes
   page_set = page_sets.KeyMobileSitesSmoothPageSet
 
   @classmethod
   def Name(cls):
     return 'v8.key_mobile_sites_smooth'
+
+class V8DetachedContextAgeInGC(benchmark.Benchmark):
+  """Measures the number of GCs needed to collect a detached context.
+
+  http://www.chromium.org/developers/design-documents/rendering-benchmarks"""
+  test = v8_detached_context_age_in_gc.V8DetachedContextAgeInGC
+  page_set = page_sets.PageReloadCasesPageSet
+
+  @classmethod
+  def Name(cls):
+    return 'v8.detached_context_age_in_gc'