navigator.userAgent not farbled properly in dynamic iframes

See third-party test page here: https://arkenfox.github.io/TZP/tzp.html

With “aggressively block fingerprinting” setting, spaces are pseudo-randomly added to navigator.userAgent. Some dynamically created iframes are producing a hash that matches the unfarbled navigator.userAgent, suggesting that farbling is not happening in those iframes.

[pilgrim-brave]

STR:

Control:

• visit https://arkenfox.github.io/TZP/tzp.html#useragent
• verify Shields are up (default) and set to “block fingerprinting” (default)
• note value next to hash field in table
• next to “iframes” click “show” to reveal 7 more hash values
• all 8 hash values should match

Test:

• change Shields to “aggressively block fingerprinting”
• note value next to hash field in table
• next to “iframes” click “show” to reveal 7 more hash values
• all 8 hash values should match

[MadhaviSeelam]

Verification PASSED using

Brave | 1.48.32 Chromium: 108.0.5359.94 (Official Build) nightly (64-bit)
-- | --
Revision | 713576b895246504ccc6b92c2fb8ce2d60194074-refs/branch-heads/5359_71@{#3}
OS | Windows 11 Version 21H2 (Build 22000.1219)

Verified using STR from https://github.com/brave/brave-browser/issues/26700#issuecomment-1339834224 and ensured that the hashes under https://arkenfox.github.io/TZP/tzp.html#useragent are matching once FP is set as Strict/Aggressive as per the following:

FP set as Standard

hash	iframes

FP set as Aggressive

hash	iframes