use muon.file.writeImportant to write ledger files

[mrose17]

Test plan

#8605 (comment)

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• Did you search for similar issues before submitting this one? yes

• Describe the issue you encountered: replace atomicWriter in ledger.js with muon.file.writeImportant

• Platform (Win7, 8, 10? macOS? Linux distro?): all

• Brave Version (revision SHA): all

• Steps to reproduce:

  1. start browser
  2. visit enough sites for enough time to cause ledger-synopsis.json to be written

• Actual result: files are written without corruption

• Expected result: same

• Will the steps above reproduce in a fresh profile? no. If not what other info can be added? visit some sites to ledger-synopsis.json gets written

• Is this an issue in the currently released version? yes

• Can this issue be consistently reproduced? yes

• Extra QA steps: none

• Screenshot if needed: none

• Any related issues: based on use write.writeImportant for session store #8593