NotEnoughFramesError no longer raises an exception.

Instead, it causes smoothness metrics to have None values. Additionaly,
the smoothness metric in smoothness.py has been refactored to more
easily support (the newly added) unit tests.

BUG=

Review URL: https://codereview.chromium.org/476363002

Cr-Commit-Position: refs/heads/master@{#292201}

tools/telemetry/telemetry/web_perf/metrics/rendering_stats.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 logging
 from operator import attrgetter
 
-from telemetry.page import page_test
 from telemetry.web_perf.metrics import rendering_frame
 
 # These are LatencyInfo component names indicating the various components
@@ -30,19 +28,6 @@ SCROLL_UPDATE_EVENT_NAME = 'InputLatency:ScrollUpdate'
 GESTURE_SCROLL_UPDATE_EVENT_NAME  = 'InputLatency:GestureScrollUpdate'
 
 
-class NotEnoughFramesError(page_test.MeasurementFailure):
-  def __init__(self, frame_count):
-    super(NotEnoughFramesError, self).__init__(
-      'Only %i frame timestamps were collected ' % frame_count +
-      '(at least two are required).\n'
-      'Issues that have caused this in the past:\n' +
-      '- Browser bugs that prevents the page from redrawing\n' +
-      '- Bugs in the synthetic gesture code\n' +
-      '- Page and benchmark out of sync (e.g. clicked element was renamed)\n' +
-      '- Pages that render extremely slow\n' +
-      '- Pages that can\'t be scrolled')
-
-
 def GetInputLatencyEvents(process, timeline_range):
   """Get input events' LatencyInfo from the process's trace buffer that are
      within the timeline_range.
@@ -139,7 +124,11 @@ class RenderingStats(object):
     if HasRenderingStats(browser_process):
       timestamp_process = browser_process
     else:
-      timestamp_process  = renderer_process
+      timestamp_process = renderer_process
+
+    # A lookup from list names below to any errors or exceptions encountered
+    # in attempting to generate that list.
+    self.errors = {}
 
     self.frame_timestamps = []
     self.frame_times = []
@@ -186,12 +175,6 @@ class RenderingStats(object):
       self._InitFrameQueueingDurationsFromTimeline(
           renderer_process, timeline_range)
 
-    # Check if we have collected at least 2 frames in every range. Otherwise we
-    # can't compute any meaningful metrics.
-    for segment in self.frame_timestamps:
-      if len(segment) < 2:
-        raise NotEnoughFramesError(len(segment))
-
   def _InitInputLatencyStatsFromTimeline(
       self, browser_process, renderer_process, timeline_range):
     latency_events = GetInputLatencyEvents(browser_process, timeline_range)
@@ -271,5 +254,5 @@ class RenderingStats(object):
       new_frame_queueing_durations = [e.queueing_duration for e in events]
       self.frame_queueing_durations.append(new_frame_queueing_durations)
     except rendering_frame.NoBeginFrameIdException:
-      logging.warning('Current chrome version does not support the queueing '
-                      'delay metric.')
+      self.errors['frame_queueing_durations'] = (
+          'Current chrome version does not support the queueing delay metric.')

tools/telemetry/telemetry/web_perf/metrics/rendering_stats_unittest.py

@@ -23,7 +23,6 @@ from telemetry.web_perf.metrics.rendering_stats import (
     ComputeInputEventLatencies)
 from telemetry.web_perf.metrics.rendering_stats import GetInputLatencyEvents
 from telemetry.web_perf.metrics.rendering_stats import HasRenderingStats
-from telemetry.web_perf.metrics.rendering_stats import NotEnoughFramesError
 from telemetry.web_perf.metrics.rendering_stats import RenderingStats
 
 
@@ -302,8 +301,10 @@ class RenderingStatsUnitTest(unittest.TestCase):
     timeline_markers = timeline.FindTimelineMarkers(['ActionA', 'ActionB'])
     timeline_ranges = [ timeline_bounds.Bounds.CreateFromEvent(marker)
                         for marker in timeline_markers ]
-    self.assertRaises(NotEnoughFramesError, RenderingStats,
-                      renderer, None, timeline_ranges)
+
+    stats = RenderingStats(renderer, None, timeline_ranges)
+    self.assertEquals(0, len(stats.frame_timestamps[1]))
+
 
   def testFromTimeline(self):
     timeline = model.TimelineModel()