Remove references to metrics removed from rendering benchmarks

Following metrics have been recently removed from rendering benchmarks:
 - percentage_smooth: Removed in http://crrev/c/2414410;
 - frame_lengths: Removed in http://crrev/c/2451211.

This CL removes references to them from rendering benchmark doc.

Bug: 894287,1135662
Change-Id: I72fd58c3c4be19e0cbb930dedd7ee6d733fac952
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2499201Reviewed-by: Jonathan Ross <jonross@chromium.org>
Commit-Queue: Mohsen Izadi <mohsen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820921}

docs/speed/benchmark/harnesses/rendering.md

@@ -30,7 +30,7 @@ These benchmarks are run on the [Chromium Perf Waterfall](https://ci.chromium.or
 Rendering metrics are [written in Javascript](https://cs.chromium.org/chromium/src/third_party/catapult/tracing/tracing/metrics/rendering/). The list of all metrics and their meanings should be documented in the files they are defined in.
 
 -   [cpu\_utilization.html](https://cs.chromium.org/chromium/src/third_party/catapult/tracing/tracing/metrics/rendering/cpu_utilization.html): `cpu_time_per_frame` and `tasks_per_frame`
--   [frame\_time.html](https://cs.chromium.org/chromium/src/third_party/catapult/tracing/tracing/metrics/rendering/frame_time.html): `frame_times`, `percentage_smooth`, `frame_lengths`, `avg_surface_fps`, `jank_count`, and `ui_frame_times`
+-   [frame\_time.html](https://cs.chromium.org/chromium/src/third_party/catapult/tracing/tracing/metrics/rendering/frame_time.html): `frame_times`, `avg_surface_fps`, `jank_count`, and `ui_frame_times`
 -   [pixels.html](https://cs.chromium.org/chromium/src/third_party/catapult/tracing/tracing/metrics/rendering/pixels.html): `mean_pixels_approximated` and `mean_pixels_checkerboarded`
 -   [queueing\_duration.html](https://cs.chromium.org/chromium/src/third_party/catapult/tracing/tracing/metrics/rendering/queueing_duration.html): `queueing_durations`