Measure paint task buffer maps in the memory profiler.

Example output from the memory profiler:

```
|       1.04 MiB -- url(http://en.wikipedia.org/wiki/Main_Page)
|          0.26 MiB -- display-list
|          0.78 MiB -- paint-task       # new output line
|             0.78 MiB -- buffer-map    # new output line
```

The buffer maps aren't huge, but they're worth measuring, and it's good
to get the memory profiler plumbing into PaintTask.

components/gfx/lib.rs

@@ -24,6 +24,7 @@ extern crate layers;
 extern crate libc;
 extern crate stb_image;
 extern crate png;
+#[macro_use]
 extern crate profile_traits;
 extern crate script_traits;
 extern crate rustc_serialize;