Crash on parsing functional notation in 'content'

[SimonSapin]

Thanks to Håkon Wium Lie for the report.

(venv)howcome@uu:~/w/2013/tests$ weasyprint http://people.opera.com/howcome/2013/tests/paged-media.html foo.pdf
Ignored `marks: crop cross` at 11:3, unknown property.
Ignored `bleed: 2mm` at 15:3, unknown property.
Ignored `position: running(header)` at 32:3, invalid value.
Ignored `flow: static(header)` at 33:3, unknown property.
Ignored `content: flow(header)` at 38:5, invalid value.
Ignored `content: element(header)` at 39:5, invalid value.
Ignored `content: flow(header)` at 57:5, invalid value.
Ignored `content: element(header)` at 58:5, invalid value.
Ignored `hyphens: auto` at 83:3, the property is experimental or non-standard, use -weasy-hyphens.
Ignored `column-span: all` at 94:3, unknown property.
Ignored `column-span: all` at 129:3, unknown property.
Ignored `column-count: 2` at 133:3, unknown property.
Ignored `-moz-column-count: 2` at 133:20, unknown property.
Ignored `column-gap: 1.5em` at 134:3, unknown property.
Ignored `-moz-column-gap: 1.5em` at 134:22, unknown property.
Ignored `column-fill: auto` at 135:3, unknown property.
Traceback (most recent call last):
  File "/home/howcome/venv/bin/weasyprint", line 9, in <module>
    load_entry_point('WeasyPrint==0.18', 'console_scripts', 'weasyprint')()
  File "/home/howcome/venv/local/lib/python2.7/site-packages/weasyprint/__main__.py", line 141, in main
    getattr(html, 'write_' + format_)(output, **kwargs)
  File "/home/howcome/venv/local/lib/python2.7/site-packages/weasyprint/__init__.py", line 159, in write_pdf
    return self.render(stylesheets).write_pdf(target, zoom)
  File "/home/howcome/venv/local/lib/python2.7/site-packages/weasyprint/__init__.py", line 131, in render
    return Document._render(self, stylesheets, enable_hinting)
  File "/home/howcome/venv/local/lib/python2.7/site-packages/weasyprint/document.py", line 252, in _render
    for css in stylesheets or []])
  File "/home/howcome/venv/local/lib/python2.7/site-packages/weasyprint/css/__init__.py", line 423, in get_all_computed_styles
    element_tree, device_media_type, url_fetcher))
  File "/home/howcome/venv/local/lib/python2.7/site-packages/weasyprint/css/__init__.py", line 179, in find_stylesheets
    url_fetcher=url_fetcher, media_type=device_media_type)
  File "/home/howcome/venv/local/lib/python2.7/site-packages/weasyprint/__init__.py", line 237, in __init__
    media_type, base_url, self.stylesheet.rules, url_fetcher))
  File "/home/howcome/venv/local/lib/python2.7/site-packages/weasyprint/css/__init__.py", line 339, in preprocess_stylesheet
    base_url, rule.declarations))
  File "/home/howcome/venv/local/lib/python2.7/site-packages/weasyprint/css/validation.py", line 1487, in preprocess_declarations
    result = list(expander_(base_url, name, tokens))
  File "/home/howcome/venv/local/lib/python2.7/site-packages/weasyprint/css/validation.py", line 1438, in validate_non_shorthand
    value = function(tokens, base_url)
  File "/home/howcome/venv/local/lib/python2.7/site-packages/weasyprint/css/validation.py", line 488, in content
    parsed_tokens = [validate_content_token(base_url, v) for v in tokens]
  File "/home/howcome/venv/local/lib/python2.7/site-packages/weasyprint/css/validation.py", line 512, in validate_content_token
    args = [a.value for a in args]
AttributeError: 'FunctionToken' object has no attribute 'value'

[SimonSapin]

The validate_content_token() function does not account for nested functions such as content: target-counter(attr(href), page).