CSSParserString::equalIgnoringCase should check string length.

The current equalIgnoringCase just checks whether the given CSSParserString is
the given str's prefix or not.

Suppose that we have the following two strings:
   char* str1; // not nul-terminated.
   int str1len; // str1's length.
   char* str2; // nul-terminated.

strncasecmp(str1, str2, str1len) == 0 doesn't mean str1 and str2 are the same string.
(Using strncasecmp instead of WTF::equalIgnoringCase.)

For example,

   str1: "abcdefg" (str1len = 7)
   str2: "abcdefghijkl\0"

so strncmp(str1, str2, str1len) returns 0.

BUG=348891
TEST=CSSParserValuesTest.EqualIgnoringCase8BitsString, CSSParserValuesTest.EqualIgnoringCase16BitsString

Review URL: https://codereview.chromium.org/189843005

git-svn-id: svn://svn.chromium.org/blink/trunk@168853 bbb929c8-8fbe-4397-9dbb-9b2b20218538