Use createStackTableBySkippingDiscarded in focusSelf.

[mstange]

Fixes #5915.

Before this fix, we were keeping the filtered-out stacks in the stack table. Now we create a stack table which has those stacks removed. A smaller stack table makes all the follow-on work faster.

Before: https://share.firefox.dev/4bXlszU
After: https://share.firefox.dev/4bJCQb6

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.42%. Comparing base (f3f07c7) to head (d199335).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5916   +/-   ##
=======================================
  Coverage   85.41%   85.42%           
=======================================
  Files         321      321           
  Lines       32157    32161    +4     
  Branches     8865     8780   -85     
=======================================
+ Hits        27468    27472    +4     
  Misses       4253     4253           
  Partials      436      436           

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