Fixed RichText placeholder whitespace.

The RichText CSS uses a rule based on attribute data-is-placeholder-visible. The TinyMce component was just setting this attribute when componentWillReceiveProps is invoked while it should also set it after the first  mount for the first props received after TinyMce setup is complete.

blocks/rich-text/tinymce.js

@@ -28,13 +28,16 @@ export default class TinyMCE extends Component {
 		return false;
 	}
 
-	componentWillReceiveProps( nextProps ) {
+	configureIsPlaceholderVisible( isPlaceholderVisible ) {
 		const name = 'data-is-placeholder-visible';
-		const isPlaceholderVisible = String( !! nextProps.isPlaceholderVisible );
-
-		if ( this.editorNode.getAttribute( name ) !== isPlaceholderVisible ) {
-			this.editorNode.setAttribute( name, isPlaceholderVisible );
+		const isPlaceholderVisibleString = String( !! isPlaceholderVisible );
+		if ( this.editorNode.getAttribute( name ) !== isPlaceholderVisibleString ) {
+			this.editorNode.setAttribute( name, isPlaceholderVisibleString );
 		}
+	}
+
+	componentWillReceiveProps( nextProps ) {
+		this.configureIsPlaceholderVisible( nextProps.isPlaceholderVisible );
 
 		if ( ! isEqual( this.props.style, nextProps.style ) ) {
 			this.editorNode.setAttribute( 'style', '' );
@@ -88,6 +91,7 @@ export default class TinyMCE extends Component {
 			setup: ( editor ) => {
 				this.editor = editor;
 				this.props.onSetup( editor );
+				this.configureIsPlaceholderVisible( this.props.isPlaceholderVisible );
 			},
 		} );
 	}