Stylesheet rule.cssText of content property should be quoted when the value is string.

LayoutTests/fast/css/csstext-of-content-string-expected.txt

+Blocked access to external URL http://example.com/
+Tests if a css text of content value is quoted when the value is string.
+
+On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
+
+
+PASS style.sheet.cssRules[0].cssText is "#A::after { content: 'A'; }"
+PASS style.sheet.cssRules[1].cssText is "#B::after { content: '\\'B\\''; }"
+PASS style.sheet.cssRules[2].cssText is "#C::after { content: '\\'C\\8\\''; }"
+PASS style.sheet.cssRules[3].cssText is "#D::after { content: '\\'D\\'', url(http://example.com/), 'EFG'; }"
+PASS successfullyParsed is true
+
+TEST COMPLETE
+