Files.app: Hide close button of share dialog.

BUG=461629
TEST=None

Review URL: https://codereview.chromium.org/975883003

Cr-Commit-Position: refs/heads/master@{#319022}

ui/file_manager/file_manager/foreground/js/ui/share_dialog.js

@@ -136,6 +136,7 @@ ShareDialog.prototype.initDom_ = function() {
   this.webViewWrapper_.className = 'share-dialog-webview-wrapper';
   this.cancelButton_.hidden = true;
   this.okButton_.hidden = true;
+  this.closeButton_.hidden = true;
   this.frame_.insertBefore(this.webViewWrapper_,
                            this.frame_.querySelector('.cr-dialog-buttons'));
 };