Move and rename RenderBR to LayoutBR.

LayoutTests/platform/mac/editing/selection/end-of-document-expected.txt

 EDITING DELEGATE: webViewDidChangeSelection:WebViewDidChangeSelectionNotification
 EDITING DELEGATE: webViewDidChangeSelection:WebViewDidChangeSelectionNotification
 EDITING DELEGATE: webViewDidChangeSelection:WebViewDidChangeSelectionNotification
 EDITING DELEGATE: webViewDidChangeSelection:WebViewDidChangeSelectionNotification
 layer at (0,0) size 800x600
   RenderView at (0,0) size 800x600
 layer at (0,0) size 800x600
   RenderBlock {HTML} at (0,0) size 800x600
     RenderBody {BODY} at (8,8) size 784x584
       RenderBlock {DIV} at (0,0) size 784x320
         RenderBlock {DIV} at (0,0) size 784x286
           RenderBlock {DIV} at (0,0) size 784x18
             RenderText {#text} at (0,0) size 311x18
               text run at (0,0) width 311: "On Feb 1, 2006, at 7:37 PM, David Hyatt wrote:"
           RenderBlock (anonymous) at (0,18) size 784x18
-            RenderBR {BR} at (0,0) size 0x18
+            LayoutBR {BR} at (0,0) size 0x18
           RenderBlock {BLOCKQUOTE} at (40,52) size 704x234 [color=#0000FF]
             RenderBlock {DIV} at (0,0) size 704x90
               RenderText {#text} at (0,0) size 449x18
                 text run at (0,0) width 449: "While reworking images I ran into something that really concerns me."
               RenderInline {SPAN} at (0,0) size 5x18
                 RenderText {#text} at (448,0) size 5x18
                   text run at (448,0) width 5: " "
               RenderText {#text} at (452,0) size 702x36
                 text run at (452,0) width 246: "WebElementImageKey is a key added"
                 text run at (697,0) width 5: " "
                 text run at (0,18) width 437: "to the elementAtPoint dictionary and the key's value is an NSImage."
               RenderInline {SPAN} at (0,0) size 4x18
                 RenderText {#text} at (437,18) size 4x18
                   text run at (437,18) width 4: " "
               RenderText {#text} at (441,18) size 263x18
                 text run at (441,18) width 263: "Unfortunately we use CGImageRef now."
               RenderInline {SPAN} at (0,0) size 1x18
                 RenderText {#text} at (703,18) size 1x18
                   text run at (703,18) width 1: " "
               RenderText {#text} at (0,36) size 701x54
                 text run at (0,36) width 697: "What this means is that as you move the mouse around a web page we are aggressively mallocing NSImages"
                 text run at (696,36) width 5: " "
                 text run at (0,54) width 674: "that then move into our image cache and stay around until the CGImageRef goes away (which could be a"
                 text run at (673,54) width 5: " "
                 text run at (0,72) width 71: "long time)."
             RenderBlock {DIV} at (0,90) size 704x18
-              RenderBR {BR} at (0,0) size 0x18
+              LayoutBR {BR} at (0,0) size 0x18
             RenderBlock {DIV} at (0,108) size 704x54
               RenderText {#text} at (0,0) size 324x18
                 text run at (0,0) width 324: "I think this is terrible but am not sure how to fix it."
               RenderInline {SPAN} at (0,0) size 5x18
                 RenderText {#text} at (323,0) size 5x18
                   text run at (323,0) width 5: " "
               RenderText {#text} at (327,0) size 205x18
                 text run at (327,0) width 205: "I assume this key is public API."
               RenderInline {SPAN} at (0,0) size 5x18
                 RenderText {#text} at (531,0) size 5x18
                   text run at (531,0) width 5: " "
               RenderText {#text} at (535,0) size 674x36
                 text run at (535,0) width 135: "Could we change the"
                 text run at (669,0) width 5: " "
                 text run at (0,18) width 264: "key's value to be a CGImageRef instead?"
               RenderInline {SPAN} at (0,0) size 5x18
                 RenderText {#text} at (263,18) size 5x18
                   text run at (263,18) width 5: " "
               RenderText {#text} at (267,18) size 664x36
                 text run at (267,18) width 393: "Could we just eliminate the key in favor of a DOM extension"
                 text run at (659,18) width 5: " "
                 text run at (0,36) width 227: "method for accessing an NSImage?"
             RenderBlock {DIV} at (0,162) size 704x18
-              RenderBR {BR} at (0,0) size 0x18
+              LayoutBR {BR} at (0,0) size 0x18
             RenderBlock {DIV} at (0,180) size 704x18
               RenderText {#text} at (0,0) size 31x18
                 text run at (0,0) width 31: "dave"
             RenderBlock {DIV} at (0,198) size 704x18
               RenderText {#text} at (0,0) size 124x18
                 text run at (0,0) width 124: "(hyatt@apple.com)"
             RenderBlock {DIV} at (0,216) size 704x18
-              RenderBR {BR} at (0,0) size 0x18
+              LayoutBR {BR} at (0,0) size 0x18
         RenderBlock (anonymous) at (0,302) size 784x18
-          RenderBR {BR} at (0,0) size 0x18
+          LayoutBR {BR} at (0,0) size 0x18
 caret: position 0 of child 0 {BR} of child 6 {DIV} of child 2 {BLOCKQUOTE} of child 1 {DIV} of child 1 {DIV} of body