[tools] Add legend to IC explorer.

Add legend that explains the abbreviations for different IC states
to IC explorer. Copied from ic.cc.

BUG=

Review-Url: https://chromiumcodereview.appspot.com/2431183004
Cr-Commit-Position: refs/heads/master@{#40433}

tools/ic-explorer.html

@@ -341,6 +341,19 @@ code is governed by a BSD-style license that can be found in the LICENSE file.
       <span style="color: #FF00FF">C</span>
       <span style="color: #00FFFF">E</span>
     </h1> Your IC-Explorer.
+
+  <div id="legend" style="padding-right: 200px">
+    <div style="float:right;  border-style: solid; border-width: 1px; padding:20px">
+    0 uninitialized<br>
+    . premonomorphic<br>
+    1 monomorphic<br>
+    ^ recompute handler<br>
+    P polymorphic<br>
+    N megamorphic<br>
+    G generic
+    </div>
+  </div>
+
   <h2>Usage</h2> Run your script with <code>--trace_ic</code> and upload on this page:<br/>
   <code>/path/to/d8 --trace_ic your_script.js > trace.txt</code>
   <h2>Data</h2>