An array of data objects can be assigned as the grid's items property.
Each object in the array represents a row in the grid's body.
The items array can be dynamically modified with Polymer's array mutation methods or by
reassigning the items property with a new array instance.
Setting height-by-rows will make <vaadin-grid> grow
and shrink in height depending on the number of rows.
The dataProvider property can be assigned a function to provide
data from a remote source.
Note: the total number of items must be set as the grid's size.
Size can be provided as the second argument of the data provider callback also.
See the "Tree Grid" demo for example.