TinyMCE: select the iframe element by id. Needed as some browser extensions insert extra elements in the page.

Props iseulde.
Merges [31180] to the 4.1 branch.
Fixes #30785.

Built from https://develop.svn.wordpress.org/branches/4.1@31436


git-svn-id: http://core.svn.wordpress.org/branches/4.1@31417 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/js/tinymce/plugins/wpeditimage/plugin.js

@@ -185,7 +185,7 @@ tinymce.PluginManager.add( 'wpeditimage', function( editor ) {
 		windowWidth = window.innerWidth;
 		toolbarWidth = toolbarNode.offsetWidth;
 		toolbarHalf = toolbarWidth / 2;
-		iframe = editor.getContentAreaContainer().firstChild;
+		iframe = document.getElementById( editor.id + '_ifr' );
 		iframePos = DOM.getPos( iframe );
 		iframeWidth = iframe.offsetWidth;
 		iframeHeigth = iframe.offsetHeight;