Fix utf-8 handling when loading URLs.

Don't use a char* to go from BString to WTF::String.
The direct conversion from BString to WTF::String assumes the data is
already UTF-8 encoded. The WTF::String constructor from char*, on the
other hand, assumes ISO-8859-15, and mangles the characters.

This gets google searches with UTF-8 characters working and fixes #7272.