Make image set insertion work in more cases

In case there was a background with multiple layers (background: url(...),
url(...);), code would just replace first URL occurrence with -webkit-image-set
rule. Handles multiple layers in case there are only url() functions. Still
won't work if value contains more values than just url()s.

Additionally, use \s for matching white space, so that Windows line endings are
also handled.

And allow line breaks between 'content|background[...]:' and the following
value. Code would previously stop matching if there was a line break between
the colon and the value.

Code review at https://codereview.chromium.org/23591022/
Patch from issue 23591022, by rchlodnicki@opera.com


git-svn-id: http://grit-i18n.googlecode.com/svn/trunk@135 7262f16d-afe8-6277-6482-052fa10e57b1