fix(common): update size error to mention 'fill' mode

The current error that is thrown when the "width" or
"height" attributes is missing doesn't mention that
"fill" mode is another option. This commit updates
the error with that option.

packages/common/src/directives/ng_optimized_image/ng_optimized_image.ts

@@ -830,7 +830,8 @@ function assertNonEmptyWidthAndHeight(dir: NgOptimizedImage) {
         `${imgDirectiveDetails(dir.ngSrc)} these required attributes ` +
             `are missing: ${missingAttributes.map(attr => `"${attr}"`).join(', ')}. ` +
             `Including "width" and "height" attributes will prevent image-related layout shifts. ` +
-            `To fix this, include "width" and "height" attributes on the image tag.`);
+            `To fix this, include "width" and "height" attributes on the image tag or turn on ` +
+            `"fill" mode with the \`fill\` attribute.`);
   }
 }