Make WebPageSerializerImpl to escape URL attribute values in result.

This patch makes |WebPageSerializerImpl| to escape URL attribute values rather
than directly output URL attribute values into result.

BUG=542054
TEST=webkit_unit_tests --gtest_filter=WebPageSerializerTest.URLAttributeValues

Review URL: https://codereview.chromium.org/1398453005

Cr-Commit-Position: refs/heads/master@{#353712}
    std::string toString() const { return m_string; }

    std::string toString() const { return m_string; }
