Rename 'composed tree' to 'flat tree' in the layout tests.

third_party/WebKit/LayoutTests/fast/dom/shadow/flat-tree-traversal-shadow-reprojection-expected.txt

 CONSOLE WARNING: Calling Element.createShadowRoot() for an element which already hosts a shadow root is deprecated. See https://www.chromestatus.com/features/4668884095336448 for more details.
 Tests for Composed Shadow DOM Tree Traversal APIs. Can only run within DRT
 
 On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
 
 
 Multiple shadow root without shadow element
-Composed Shadow Tree:
+Flat Tree:
 #a
         #c
 
 Traverse in forward.
 #a
 #c
 Traverse in backward.
 #c
 #a
 
 Multiple shadow root with shadow element
-Composed Shadow Tree:
+Flat Tree:
 #a
         #c
         #b
         #e
 
 Traverse in forward.
 #a
 #c
 #b
 #e
 Traverse in backward.
 #e
 #b
 #c
 #a
 
 Multiple shadow root with shadow element, case 2
-Composed Shadow Tree:
+Flat Tree:
 #a
         #f
         #c
         #b
         #e
         #h
 
 Traverse in forward.
 #a
 #f
 #c
 #b
 #e
 #h
 Traverse in backward.
 #h
 #e
 #b
 #c
 #f
 #a
 
 shadow reprojection
-Composed Shadow Tree:
+Flat Tree:
 #a
         #b
                 #d
                 #a-child
                 #f
 
 Traverse in forward.
 #a
 #b
 #d
 #a-child
 #f
 Traverse in backward.
 #f
 #a-child
 #d
 #b
 #a
 
 shadow reprojection, take 2
-Composed Shadow Tree:
+Flat Tree:
 #a
         #d
                 #f
                 #b
                 #h
 
 Traverse in forward.
 #a
 #d
 #f
 #b
 #h
 Traverse in backward.
 #h
 #b
 #f
 #d
 #a
 
 Ignore shadow fallback elements
-Composed Shadow Tree:
+Flat Tree:
 #a
         #i
                 #k
                 #c
                 #b
                 #f
                         #g
                 #h
                 #m
 
@@ skipping 12 matching lines @@
 #h
 #g
 #f
 #b
 #c
 #k
 #i
 #a
 
 with inactive insertion points
-Composed Shadow Tree:
+Flat Tree:
 #a
         #s
                 #u
                 #f
                 #b
                         #c
                 #d
                         #e
                 #l
                 #m
@@ skipping 24 matching lines @@
 #b
 #f
 #u
 #s
 #a
 
 PASS successfullyParsed is true
 
 TEST COMPLETE