Add llvm to macOS required dependencies in docs
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@gterzian described it [in their comment](https://github.com/servo/servo/issues/21498#issuecomment-415698332) pretty well:
> latest "command line tools" that come with it(and include Apple's version of llvm), it might be that the current default Mac llvm is too old to compile the latest mozjs version(it appeared that after the OS update, no version of clang/llvm was installed until I downloaded the "new" command-line tools for the new OS)
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Mention gstreamer LIB variable for Windows
This got missed.
Ideally we'll get bootstrap to do all of this, but I haven't yet made
bootstrap work properly for Windows.
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More servo-tidy.toml cleanup
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There are a few mentions of nsRuleNode left but they are mostly
historical references so it makes sense to keep them.
Differential Revision: https://phabricator.services.mozilla.com/D5505
Respect CSS border-image-width.
Properly support gradients as a border-image-source.
Only emit a border item if the border-width is non-zero
for simple borders but still emit one if the item is
an image as paint worklet that are not drawn cause servo
to hang and fail tests.
Add a new test and mark 4 more as passing.
Make tests for uninitialized renderbuffers pass
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- [x] These changes fix#13710
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Fix private browsing mode preservation when loading URL
`Constellation::load_url` disregards the existing `BrowsingContext`'s `is_private` field when loading a new URL and creating a new `Pipeline` for the page load, which means that the `Pipeline` is always passed the public resource thread, even if its browsing context is in private browsing mode. This PR fixes that.
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- [x] I'm not actually sure. I haven't found tests related to private browsing so maybe that's why this bug wasn't caught earlier?
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Create ScriptMsg::GetBrowsingContextInfo
Script used to send two messages to constellation to first retrieve the id of the browsing context in which a pipeline resides and then its parent pipeline's id. This patch introduces a minor optimization to instead retrieve those fields in a single message.
Also, fixed a potential bug introduced in #21559 where if a browsing context wasn't found for a pipeline in response to `GetParentInfo` (which is now deleted), constellation sent nothing back, presumably causing the script thread to hang on the receiving channel.
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