[WebProfilerBundle] Fixed profiler toolbar icons for XHTML.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | n/a
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

Profiler page itself is entirely autonomic, but toolbar is not - unfortunately currently it doesn't work in XHTML documents :(.

#9877 backported to `2.3` branch.