Added support to backtrace from botton of stack to debugger protocol

test/mjsunit/debug-backtrace.js

Index: test/mjsunit/debug-backtrace.js
===================================================================
--- test/mjsunit/debug-backtrace.js	(revision 1840)
+++ test/mjsunit/debug-backtrace.js	(working copy)
@@ -124,6 +124,23 @@
     assertEquals(2, frames[1].index);
     assertEquals("", response.lookup(frames[1].func.ref).name);
 
+    // Get backtrace with bottom two frames.
+    json = '{"seq":0,"type":"request","command":"backtrace","arguments":{"fromFrame":0,"toFrame":2, "bottom":true}}'
+    response = new ParsedResponse(dcp.processDebugJSONRequest(json));
+    backtrace = response.body();
+    assertEquals(1, backtrace.fromFrame);
+    assertEquals(3, backtrace.toFrame);
+    assertEquals(3, backtrace.totalFrames);
+    var frames = backtrace.frames;
+    assertEquals(2, frames.length);
+    for (var i = 0; i < frames.length; i++) {
+      assertEquals('frame', frames[i].type);
+    }
+    assertEquals(1, frames[0].index);
+    assertEquals("g", response.lookup(frames[0].func.ref).name);
+    assertEquals(2, frames[1].index);
+    assertEquals("", response.lookup(frames[1].func.ref).name);
+
     // Get the individual frames.
     json = '{"seq":0,"type":"request","command":"frame"}'
     response = new ParsedResponse(dcp.processDebugJSONRequest(json));