LibWeb: Start working on supporting fixed table layouts

Sometimes people make tables with a specific width. In those cases,
we can't just use the auto-sizing algorithm, but instead have to
respect whatever width the content specifies.

This is a bit rickety right now, since we don't implement generation
of anonymous table boxes.

The basic mechanism here is that block layout (which table-cell uses)
now has a virtual way of asking for the width of the logical containing
block. This is necessary as table-row does not produce a block-level
element and so was previously unable to provide a containing block
width for table-cell layout.

If the table has a non-auto specified width, we now interpret that as
a request to use fixed table layout. This will evolve over time. :^)

Libraries/LibWeb/Layout/LayoutTableCell.h

@@ -43,6 +43,7 @@ public:
 private:
     virtual bool is_table_cell() const override { return true; }
     virtual const char* class_name() const override { return "LayoutTableCell"; }
+    virtual float width_of_logical_containing_block() const override;
 };
 
 template<>