[ios] Ensure MDCAppBarViewController matches the width of ChromeTableViewController.

Late addition to the docs: https://github.com/material-components/material-components-ios/tree/develop/components/AppBar#migration-guide-mdcappbar-to-mdcappbarviewcontroller

Bug: 874471
Change-Id: Ic8605f56a60f5deeaf316a88a8291540364604c1
Reviewed-on: https://chromium-review.googlesource.com/1239445Reviewed-by: Sergio Collazos <sczs@chromium.org>
Commit-Queue: Chris Lu <thegreenfrog@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593383}

ios/chrome/browser/ui/table_view/chrome_table_view_controller.mm

@@ -86,6 +86,11 @@
     self.appBarViewController.headerView.trackingScrollView = self.tableView;
     // Add the AppBar's views after all other views have been registered.
     [self addChildViewController:_appBarViewController];
+    CGRect frame = self.appBarViewController.view.frame;
+    frame.origin.x = 0;
+    frame.size.width =
+        self.appBarViewController.parentViewController.view.bounds.size.width;
+    self.appBarViewController.view.frame = frame;
     [self.view addSubview:self.appBarViewController.view];
     [self.appBarViewController didMoveToParentViewController:self];
   }