Without this `./mach run -b -- -M` would start fine but navigating to a site would fail with:
`thread 'ScriptThread PipelineId { namespace_id: PipelineNamespaceId(0), index: PipelineIndex(0) }' panicked at 'assertion failed: mozbrowser_enabled()', /Users/ulf/Documents/Code/servo/components/script/dom/htmliframeelement.rs:163`
This adds in preferences for all the SM 39 available options (as
retrieved from Gecko), and uses the same defaults as Gecko. A few
properties are not supported yet, and incremental GC is still always
disabled regardless of the preference setting.
This also adds back in the options that were accidentally deleted when
\#10342 was rebased, which moved things from script_thread.rs to
script_runtime.rs.
This allows both boolean and string-type preferences. It
also implements a system where prefs that are read from a
configuration file can be reset back to their initial value,
which is useful in a number of cases e.g. when running tests
to ensure that each test starts with the same values for
the prefs.
