Add support for Float/Double types for JS Mojo bindings.

content/test/data/web_ui_test_mojo_bindings.mojom

 module mojo {
 
 struct EchoArgs {

[sky, 2014/04/23 22:37:05]

Please document what this is for and what the values are intended for.

   int64 si64;
   int32 si32;
   int16 si16;
   int8  si8;
   uint64 ui64;
   uint32 ui32;
   uint16 ui16;
   uint8  ui8;
+  float float_val;
+  float float_inf;
+  float float_nan;
+  double double_val;
+  double double_inf;
+  double double_nan;
   string name;
 };
 
 [Peer=RendererTarget]
 interface BrowserTarget {
   PingResponse();
   EchoResponse(EchoArgs arg1, EchoArgs arg2);
 };
 
 [Peer=BrowserTarget]
 interface RendererTarget {
   Ping();
   Echo(EchoArgs arg);
 };
 
 }