[css-grid-3][masonry] Update webm example videos to mp4 (w3c#9536)

Author: stwrt

css-grid-3/Overview.bs

@@ -554,7 +554,7 @@ Fragmentation in the Masonry Axis</h3>
 		illustrating fragmentation of a grid with masonry layout in its [=block axis=].
 		It renders like this:
 		<figure style="max-width:100%">
-			<video style="max-width:100%" src="images/fragmentation-block-axis-example.webm" controls></video>
+			<video style="max-width:100%" src="images/fragmentation-block-axis-example.mp4" controls></video>
 			<figcaption>
 				Visualization of fragmentation in a [=block-axis=] [=masonry layout=].
 			</figcaption>
@@ -577,7 +577,7 @@ Fragmentation in the Grid Axis</h3>
 		In this case the breaks occurs between the [=grid-axis=] rows.
 		It renders like this:
 		<figure style="max-width:100%">
-			<video style="max-width:100%" src="images/fragmentation-inline-axis-example.webm" controls></video>
+			<video style="max-width:100%" src="images/fragmentation-inline-axis-example.mp4" controls></video>
 			<figcaption>
 				Visualization of fragmentation in the [=block axis=] with [=inline-axis=] [=masonry layout=].
 			</figcaption>
@@ -736,7 +736,7 @@ Graceful Degradation</h2>
 		Here's what it looks like with Masonry support for comparison:
 
 		<figure>
-			<video src="images/graceful-degradation-example.webm" controls></video>
+			<video src="images/graceful-degradation-example.mp4" controls></video>
 			<figcaption>
 				Rendering of the example in a UA with Masonry support.
 			</figcaption>