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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Honor SERVO_ENABLE_DEBUG_ASSERTIONS on the build machines.
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As part of #13127.
cc @aneeshusa
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Use parking_lot::RwLock for PropertyDeclarationBlock
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As discussed in https://bugzilla.mozilla.org/show_bug.cgi?id=1305141Closes#13176
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Original PR title: Stop relying on `impl<T: HeapSizeOf> HeapSizeOf for Arc<T>`
https://github.com/servo/heapsize/issues/37#issuecomment-249861171
This builds on top of that.
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Make readystatechange fire more often (fixes#13481)
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