[telemetry] Implement first version of timeline based measurement

tools/perf/metrics/timeline_based_metric.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.
+
+class TimelineBasedMetric(object):
+  def __init__(self):
+    """Computes metrics from a telemetry.core.timeline Model and a range
+
+    """
+    super(TimelineBasedMetric, self).__init__()
+
+  def AddResults(self, model, renderer_thread,
+                 metric_request, results):
+    """ Override to actually add metrics to the results. Metric that overrides
+    this method should only compute metric on the data between time_bound.
+
+    model is a model of timeline APIT. (TODO: make a shared timeline superclass)
+    meric_request is an instance of TimelineBasedMetricRequest
+    results is an instance of page.PageTestResults.
+    """
+    raise NotImplementedError()