[telemetry] Add a V8GCTimes measurement and benchmarks.

Replaces the v8_stats metric with a v8_gc_times
measurement.  This new measurement measures the amount of
time spent in GC during the whole benchmark rather than
just during gestures.

The following gc metrics are measured:
  v8_gc_incremental_marking
  v8_gc_incremental_marking_idle_deadline_overrun
  v8_gc_incremental_marking_outside_idle
  v8_gc_incremental_marking_percentage_idle
  v8_gc_mark_compactor
  v8_gc_mark_compactor_idle_deadline_overrun
  v8_gc_mark_compactor_outside_idle
  v8_gc_mark_compactor_percentage_idle
  v8_gc_scavenger
  v8_gc_scavenger_idle_deadline_overrun
  v8_gc_scavenger_outside_idle
  v8_gc_scavenger_percentage_idle
  v8_gc_total
  v8_gc_total_idle_deadline_overrun
  v8_gc_total_outside_idle

It also measures the duration of the measurement and the
amount of CPU time used.

The v8_garbage_collection_cases benchmark is modified to
use this new v8_gc_times measurement, and v8.top_25_smooth
and v8.key_mobile_sites_smooth benchmarks have been added.

Since v8_garbage_collection_cases was the only benchmark to
use the is_fast metric type, and these measurements are now
done as part of the v8_gc_times measurement now, this CL
also removes the fast metric type.

Perf Sherrifs: Since this CL changes he way in which the GC
measurements are done it may cause a movement in the
v8_garbage_collection_cases benchmarks.

Committed: https://crrev.com/452de5927e54e1da0d57a4b2df8b447f4d442360
Cr-Commit-Position: refs/heads/master@{#314868}

Committed: https://crrev.com/e65bf3eeb5227de0bb05f08169719d06e518de8b
Cr-Commit-Position: refs/heads/master@{#315550}

Review URL: https://codereview.chromium.org/898673005

Cr-Commit-Position: refs/heads/master@{#315756}

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