Telemetry: Re-enable slide_drawer in key_silk_cases

This test had been disabled because of failures in
smoothness.key_silk_cases, but these failures are no
longer reproducible locally or on the perf trybots.

BUG=404317

Review URL: https://codereview.chromium.org/791183002

Cr-Commit-Position: refs/heads/master@{#308464}

tools/perf/page_sets/key_silk_cases.py

@@ -676,8 +676,7 @@ class KeySilkCasesPageSet(page_set_module.PageSet):
     self.AddUserStory(Page16(self))
     self.AddUserStory(Page17(self))
     self.AddUserStory(Page18(self))
-    # crbug.com/404317
-    # self.AddUserStory(Page19(self))
+    self.AddUserStory(Page19(self))
     self.AddUserStory(Page20(self))
     self.AddUserStory(GwsGoogleExpansion(self))
     self.AddUserStory(GwsBoogieExpansion(self))