Bug 1303229: Get the proper viewport size for stylo. r=heycam

At least until we support scrollbars properly, this size is going to be the
correct one. I've left a TODO to grab the proper one once we support it.

This allows to trivially test viewport units for now.

MozReview-Commit-ID: JdaZ6WlZ2C6
Signed-off-by: Emilio Cobos Álvarez <emilio@crisal.io>

components/layout/multicol.rs

@@ -140,7 +140,7 @@ impl Flow for MulticolFlow {
                     // FIXME: (until column balancing) substract margins/borders/padding
                     LogicalSize::from_physical(
                         self.block_flow.base.writing_mode,
-                        ctx.shared_context().viewport_size,
+                        ctx.shared_context().viewport_size(),
                     ).block
                 }
             }