[WebProfilerBundle] Show AJAX requests in the symfony profiler toolbar

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Adds AJAX requests in the web debug toolbar.
See #8896 for the original discussion.

![image](https://cloud.githubusercontent.com/assets/47313/4372525/e6a0bcbe-4324-11e4-9ab8-9d6aec1e4316.png)
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