Revert "[tools/perf] Disable youtube_pinch_2018 in smoothness benchmarks"

This reverts commit 076b591c.

Reason for revert: Root cause has been fixed.

Original change's description:
> [tools/perf] Disable youtube_pinch_2018 in smoothness benchmarks
> 
> Stories have been consinstently failing on N5X and Pixel 2 bots.
> 
> TBR=nednguyen@google.com
> NOTRY=true
> 
> Bug: 875859
> Change-Id: I9d41fa293a90e6961eabc6b29711d8b824d469ce
> Reviewed-on: https://chromium-review.googlesource.com/1181140
> Reviewed-by: Juan Antonio Navarro Pérez <perezju@chromium.org>
> Commit-Queue: Juan Antonio Navarro Pérez <perezju@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#584440}

TBR=perezju@chromium.org,nednguyen@google.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 875859
Change-Id: Id8caf496237be5e80d9d0d6dbe36554e722f6689
Reviewed-on: https://chromium-review.googlesource.com/1186481Reviewed-by: Juan Antonio Navarro Pérez <perezju@chromium.org>
Commit-Queue: Juan Antonio Navarro Pérez <perezju@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585463}

tools/perf/expectations.config

@@ -181,14 +181,6 @@ crbug.com/815193 [ Android ] rasterize_and_record_micro.top_25/file://static_top
 crbug.com/873011 [ Android_Webview ] rasterize_and_record_micro.top_25/file://static_top_25/yahoonews.html [ Skip ]
 crbug.com/865400 [ Pixel_2 ] rasterize_and_record_micro.top_25/file://static_top_25/yahoonews.html [ Skip ]
 
-# Benchmark: smoothness.tough_pinch_zoom_cases
-crbug.com/875859 [ Nexus_5X ] smoothness.tough_pinch_zoom_cases/youtube_pinch_2018 [ Skip ]
-crbug.com/875859 [ Pixel_2 ] smoothness.tough_pinch_zoom_cases/youtube_pinch_2018 [ Skip ]
-
-# Benchmark: smoothness.gpu_rasterization.tough_pinch_zoom_cases
-crbug.com/875859 [ Nexus_5X ] smoothness.gpu_rasterization.tough_pinch_zoom_cases/youtube_pinch_2018 [ Skip ]
-crbug.com/875859 [ Pixel_2 ] smoothness.gpu_rasterization.tough_pinch_zoom_cases/youtube_pinch_2018 [ Skip ]
-
 # Benchmark: system_health.common_desktop
 crbug.com/828917 [ Mac ] system_health.common_desktop/multitab:misc:typical24 [ Skip ]
 crbug.com/874803 [ Win_10 ] system_health.common_desktop/multitab:misc:typical24 [ Skip ]