Inline the init method in the StructTreeRoot constructor

[Snuffleupagus]

Currently the constructor only set various class fields and the class instance thus needs to be "manually" initialized, which seems unnecessary.
Given how short/simple the init and readRoleMap methods are we can just inline their code in the constructor, thus simplifying the code overall.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.80%. Comparing base (86ffd68) to head (8232440).

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #21505      +/-   ##
==========================================
- Coverage   89.82%   89.80%   -0.03%     
==========================================
  Files         260      260              
  Lines       66274    66267       -7     
==========================================
- Hits        59532    59512      -20     
- Misses       6742     6755      +13     

Flag	Coverage Δ
browsertest	66.70% <100.00%> (+0.01%)	⬆️
fonttest	9.03% <ø> (ø)
integrationtest	68.94% <100.00%> (-0.01%)	⬇️
unittest	57.22% <100.00%> (-0.02%)	⬇️
unittestcli	56.20% <100.00%> (+0.01%)	⬆️

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[Snuffleupagus]

/botio browsertest

[moz-tools-bot]

From: Bot.io (Linux m4)

---

Received

Command cmd_browsertest from @Snuffleupagus received. Current queue size: 0

Live output at: http://54.241.84.105:8877/52ec29189a3ad86/output.txt

[moz-tools-bot]

From: Bot.io (Windows)

---

Received

Command cmd_browsertest from @Snuffleupagus received. Current queue size: 0

Live output at: http://54.193.163.58:8877/d0686b52f0a67bd/output.txt

[moz-tools-bot]

From: Bot.io (Linux m4)

---

Success

Full output at http://54.241.84.105:8877/52ec29189a3ad86/output.txt

Total script time: 18.11 mins

• Regression tests: Passed

[moz-tools-bot]

From: Bot.io (Windows)

---

Success

Full output at http://54.193.163.58:8877/d0686b52f0a67bd/output.txt

Total script time: 25.12 mins

• Regression tests: Passed

[timvandermeij]

Thank you for simplifying this code!