Remove frame flattening support as Chromium has no intention of using it

LayoutTests/TestExpectations

 # See http://trac.webkit.org/wiki/TestExpectations for more information on this file.
 
 # -----------------------------------------------------------------
 # SLOW TESTS
 # -----------------------------------------------------------------
 
 crbug.com/24182 [ Debug ] css3/flexbox/multiline-justify-content.html [ Slow ]
 webkit.org/b/91666 [ Mac ] editing/input/emacs-ctrl-o.html [ Failure Slow ]
 webkit.org/b/91666 [ Mac ] editing/pasteboard/emacs-ctrl-k-y-001.html [ Failure Slow ]
 webkit.org/b/91666 [ Mac ] editing/pasteboard/emacs-ctrl-a-k-y.html [ Failure Slow ]
@@ skipping 289 matching lines @@
 # These tests try to print a PDF file as the expected result.  I don't think
 # we plan on supporting this anytime soon.
 printing/compositing-layer-printing.html [ WontFix ]
 printing/media-queries-print.html [ WontFix ]
 printing/page-rule-in-media-query.html [ WontFix ]
 
 # Repaint test doesn't seem to work on Chromium.
 webkit.org/b/98579 compositing/repaint/requires-backing-repaint.html [ ImageOnlyFailure ]
 webkit.org/b/98579 virtual/softwarecompositing/repaint/requires-backing-repaint.html [ ImageOnlyFailure ]
 
-# Tests for frameset flattening which is not used by Chromium.
-fast/text-autosizing/narrow-iframe-flattened.html [ WontFix ]
-http/tests/misc/iframe-flattening-3level-nesting-with-blocking-resource.html [ WontFix ]
-plugins/frameset-with-plugin-frame.html [ WontFix ]
-
 # Chromium's video codecs don't support alpha information encoded in the video
 # data, so this test is not applicable.
 compositing/video/video-background-color.html [ WontFix ]
 virtual/softwarecompositing/video/video-background-color.html [ WontFix ]
 
 # Chrome uses its quota manager and doesn't respect the 5mb limit.
 storage/indexeddb/database-quota.html [ WontFix ]
 
 # Chromium doesn't use fullscreen-specific media controls
 media/video-controls-fullscreen-volume.html [ WontFix ]
@@ skipping 2811 matching lines @@
 # From WebKit's generic expectations.
 # pending functional patch and per-port verification
 webkit.org/b/109954 css3/line-break [ Skip ]
 # pending implementation completion and feature enabling
 webkit.org/b/109570 media/track/regions-webvtt [ Skip ]
 
 crbug.com/231721 [ XP Release ] svg/custom/invisible-text-after-scrolling.xhtml [ Pass Timeout ]
 
 # The image output is wrong (some red dots can be seen).
 crbug.com/232061 [ Linux ] fast/text/zero-font-size-2.html [ ImageOnlyFailure ]