Move PropertyDeclarationBlock into its own module.
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`properties.mako.rs` (its previous location) is a fairly big file.
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Run CSS Multicolumn tests automatically
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Implements the CSS multicolumn tests as outlined in issue #13587.
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Removed the session history from BrowsingContext.
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The session history is stored in the constellation and accessed via the `History` object, so `BrowsingContext` doesn't need to track it.
cc @jdm @ConnorGBrewster @Ms2ger
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Invert conditional and return early for AddEventListener/RemoveEventL…
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Cleans up code as described in #13586 by returning early instead of nesting everything in an `if` statement.
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…istener
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Ensure that SERVO_ENABLE_DEBUG_ASSERTIONS is also used in build-cef
r? @nox
Fixes#13591
This fixes the problem with rebuilding all of CEF on the linux-rel builders by ensuring it gets the same `RUSTFLAGS`. Proof:
```
[larsberg@larsberg servo2]$ ./mach build-cef -r
Compiling embedding v0.0.1 (file:///Users/larsberg/servo2/ports/cef)
Finished release [optimized] target(s) in 35.42 secs
[Warning] Could not generate notification! Optional Python module 'pyobjc' is not installed.
CEF build completed in 0:00:38
[larsberg@larsberg servo2]$
```
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