Telemetry: preparation for deleting timeline.py

We have TBMv2 versions of thread_times metrics, implemented in the
Catapult repo in tracing/tracing/metrics/rendering. The legacy code
for these metrics live in the Chromium repo in
tools/perf/metrics/timeline.py.

In order to not have a pause in the perf dashboard when we replace
metrics defined in the Chromium repo with the ones defined in the
Catapult repo, we plan to:

1- (this CL) modify the AddTBMv2RenderingMetrics function to behave
   when both metrics (with the same name) exist,

2- for all (groups of) thread times metrics:
2.1- rename the TBMv2 version, in the Catapult repo, to have the same
     name as the legacy metric,
2.2- delete the legacy metric, from the Chromium repo.

3- and, finally, delete the rendering measurement and rendering_util,
   and completely move to TBMv2.

Bug: 890757
Change-Id: Iafb84d3ec0f732177005651e0b54b8dccb307c65
Reviewed-on: https://chromium-review.googlesource.com/c/1307838Reviewed-by: Ned Nguyen <nednguyen@google.com>
Commit-Queue: Ehsan Chiniforooshan <chiniforooshan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604592}

tools/perf/measurements/rendering_util.py

@@ -14,10 +14,12 @@ def AddTBMv2RenderingMetrics(trace_value, results, import_experimental_metrics):
   for f in mre_result.failures:
     results.Fail(f.stack)
 
+  existing_values = {v.name for v in results.all_page_specific_values}
   histograms = []
   for histogram in mre_result.pairs.get('histograms', []):
-    if (import_experimental_metrics or
-        histogram.get('name', '').find('_tbmv2') < 0):
+    name = histogram.get('name', '')
+    if ((import_experimental_metrics or name.find('_tbmv2') < 0) and
+        not name in existing_values):
       histograms.append(histogram)
   results.ImportHistogramDicts(histograms, import_immediately=False)