The content process stores the incoming initial gfxVars updates, which are
lazily used when the gfxVars are first initialized.
MozReview-Commit-ID: ExUVdr5xGLb
When a subprocess is launched, gfxVars updates (for non-default values) are
serialized and passed on the command line, up to a limit of 1023 characters,
and ensuring it should not overflow the command line size.
When the child starts, the command line parameter is given to gfxVars, so the
updates can be used during gfxVars::Initialize(), instead of doing a sync
request to the parent.
In case the updates are not sent, or in the unlikely case the child cannot
parse them, we fallback to the sync request -- The former case should be rare
enough that a slow sync request is acceptable: It should only happen if D3D
block-list is *modified* (most people would either use the default, or just
overwrite these prefs with short strings.)
MozReview-Commit-ID: 6MoJC0fe59Q
