fix PDF pinned tabs being duplicated

[diracdeltas]

fix #9954

Test Plan:
npm run test -- --grep='Pins an existing frame'
try pinning a PDF frame. it should not be duplicated

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.

Test Plan:

Reviewer Checklist:

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[codecov-io]

Codecov Report

Merging #10116 into master will increase coverage by <.01%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master   #10116      +/-   ##
==========================================
+ Coverage   52.77%   52.77%   +<.01%     
==========================================
  Files         227      227              
  Lines       20138    20139       +1     
  Branches     3223     3223              
==========================================
+ Hits        10628    10629       +1     
  Misses       9510     9510

Flag	Coverage Δ
#unittest	52.77% <50%> (ø)	⬆️

Impacted Files	Coverage Δ
app/browser/windows.js	17.89% <50%> (+0.32%)	⬆️

[cezaraugusto]

++ thanks for the test

[bsclifton]

Awesome, great job on this 😄