Specify that grid areas are definite after track sizing is done. Fixes #1319. Fixes #1320.

Author: tabatkins

css-grid/Overview.bs

@@ -3554,6 +3554,13 @@ Grid Sizing Algorithm</h3>
 
 	Once the size of each <a>grid area</a> is thus established,
 	the <a>grid items</a> are laid out into their respective containing blocks.
+	The <a>grid area’s</a> width and height are considered <a>definite</a> for this purpose.
+
+	Note: Since formulas calculated using only definite sizes,
+	such as the <a>stretch fit</a> formula,
+	are also definite,
+	the size of a grid item which is stretched
+	is also considered definite.
 
 <h3 id='algo-terms'>
 Track Sizing Terminology</h3>