Refactor call tree search to support both Code and Function trees

runtime/observatory/lib/src/elements/cpu_profile.html

 <link rel="import" href="../../../../packages/polymer/polymer.html">
 <link rel="import" href="code_ref.html">
 <link rel="import" href="function_ref.html">
 <link rel="import" href="nav_bar.html">
 <link rel="import" href="observatory_element.html">
 <link rel="import" href="sliding_checkbox.html">
 <link rel="import" href="view_footer.html">
 
 <polymer-element name="sample-buffer-control" extends="observatory-element">
   <template>
@@ skipping 468 matching lines @@
     <style>
       .tableWell {
         background-color: #ECECEC;
         padding: 0.2em;
       }
     </style>
     <sample-buffer-control id="sampleBufferControl"></sample-buffer-control>
     <br>
     <stack-trace-tree-config id="stackTraceTreeConfig"></stack-trace-tree-config>
     <br>
-    <cpu-profile-virtual-tree class="shadow" id="cpuProfileVirtualTree"></cpu-profile-virtual-tree>
+    <cpu-profile-virtual-tree id="cpuProfileVirtualTree"></cpu-profile-virtual-tree>
   </template>
 </polymer-element>
 
 <script type="application/dart" src="cpu_profile.dart"></script>