Auto merge of #20407 - dbaron:bug1448138, r=erahm

Rename Gecko string DataFlags::SHARED to REFCOUNTED

This is the servo piece of:

[Bug 1448138](https://bugzilla.mozilla.org/show_bug.cgi?id=1448138) - Rename string DataFlags::SHARED to REFCOUNTED to make it clearer to those reading the code.  r=erahm

This is a renaming in the internals of Gecko's string library that should also have a corresponding rename in the rust bindings.  This is already reviewed as part of the Gecko change, and the two pieces should be able to land separately.

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