Change profile.shared.stackTable.prefix to prefixOffset (delta from current index)

[mstange]

Two new versions on top of #6087:

• v66: Change stackTable.prefix to an Int32Array (with -1 instead of null for root nodes)
• v67: Change stackTable.prefix to prefixOffset (with 0 for root nodes and i - prefix[i] offsets for non-roots)

Here's how this change impacts profile sizes and loading times on this profile: https://storage.googleapis.com/profiler-get-symbols-fixtures/large-speedometer3-profile.json.gz (with this corresponding size profile)

Version	.jslb.gz size	.jslb size	Load time	Profile of it loading
64	122 MB	605 MB	7.6 seconds	https://share.firefox.dev/4ogUKba
65	125 MB	544 MB	6.0 seconds	https://share.firefox.dev/3Qopiem
66	130 MB	468 MB	5.7 seconds	https://share.firefox.dev/4uePEgU
67	89 MB	468 MB	5.3 seconds	https://share.firefox.dev/49JetKx

125 MB -> 89 MB on the compressed size! That's a 29% reduction!

[codecov]

Codecov Report

❌ Patch coverage is 89.00000% with 22 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.30%. Comparing base (53a3071) to head (610f1a9).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/profile-logic/profile-compacting.ts	57.44%	20 Missing ⚠️
src/profile-logic/transforms.ts	95.91%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6089      +/-   ##
==========================================
- Coverage   83.35%   83.30%   -0.05%     
==========================================
  Files         339      339              
  Lines       35941    36048     +107     
  Branches    10058     9997      -61     
==========================================
+ Hits        29957    30030      +73     
- Misses       5556     5590      +34     
  Partials      428      428              

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[mstange]

This is now ready for review.

@canova, how do you feel about this? People will have to change any code that generates profiles, but I actually think the changes would be pretty straightforward and non-controversial.

Also, I can fold the two versions into one if you'd prefer that, so that we don't have an intermediate version that uses -1 values.

[mstange]

When we compute the derived stack table, we turn prefixOffset back into prefix. We could shave off another few milliseconds if the derived stack table also switched to the prefixOffset format, then we wouldn't need to do the conversion. We can do that as a follow-up if we think it's worth doing.