Use non-persistent session for requests by default

[diracdeltas]

fix #12469

Test Plan:

1. Turn on Brave payments in a fresh profile. Everything should work.
2. Click 'check for updates'. It should not show console errors. If in a packaged build, it should say 'no updates available'.

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.
• Request a security/privacy review as needed. (Ask a Brave employee to help if you cannot access this document.)

Test Plan:

Reviewer Checklist:

• Request a security/privacy review as needed if one was not already requested.

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 #12694 into master will decrease coverage by <.01%.
The diff coverage is 28.57%.

@@            Coverage Diff             @@
##           master   #12694      +/-   ##
==========================================
- Coverage   56.09%   56.09%   -0.01%     
==========================================
  Files         279      279              
  Lines       27302    27307       +5     
  Branches     4441     4442       +1     
==========================================
+ Hits        15316    15318       +2     
- Misses      11986    11989       +3

Flag	Coverage Δ
#unittest	56.09% <28.57%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
js/lib/request.js	17.64% <28.57%> (+2.42%)	⬆️

[bridiver]

maybe let defaultSession = getDefaultSession() would be better than setDefaultSession method with side effects?

[bsclifton]

Per our convo, could it be updated to be more of a getter? ex:

getDefaultSession = function () {
  if (!defaultSession){
    defaultSession = session.fromPartition('default')
  }
  return defaultSession
}

const session = getDefaultSession()

[bridiver]

++

[bsclifton]

Changes look great! 😄

[diracdeltas]

0.21.x: 375cd9f
master: 735b4ef