speedreader: Make heuristics backend feature flag

[kkuehlz]

This is more accessible for my team than a command line switch.

Resolves brave/brave-browser#15179

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• Ran git rebase master (if needed)

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• Major classes, functions and non-trivial code blocks are well-commented
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After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
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  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

[pes10k]

@keur just to make sure I understand, can you clarify a couple of things?

1. I see two flags (Speedreeder and Speedreader Heuristics). How do these interact? If i want the "Heuristics" version, do i enable both? Or only the "Speedreader Heuristics" flag?
2. I still think these names are confusing (they're both using heuristics!). WDYT of "Speedreeder - Rules List" and "Speedreeder - Readability" or similar?

[kkuehlz]

@pes10k yeah. so the speedreader flag is the flag for the speedreader feature itself (it was already present. it's just included in this diff because i am adding the new feature flag below it). the speedreader heuristics backend switches to the readability backend we are using. and yeah sorry about the names i'll change it.

Since the "Rules List" is the default already we don't include a flag for that.

[pes10k]

thanks @keur !

[kkuehlz]

note: this already passed CI, just did a name change for some variables and they timed out. unrelated.

[iefremov]

needs a space before {

[kkuehlz]

clang-format did this. i agree it looks kinda wonky, but it shows up in other files too

[iefremov]

looks like bad spacing, needs clang-format ?

[kkuehlz]

clang-format also did this

[mkarolin]

chromium_src changes LGTM