Escape "--" in the page URL at page serialization

This patch makes page serializer to escape the page URL embed into a HTML
comment of result HTML[1] to avoid inserting text as HTML from URL by
introducing a static member function |PageSerialzier::markOfTheWebDeclaration()|
for sharing it between |PageSerialzier| and |WebPageSerialzier| classes.

[1] We use following format for serialized HTML:
saved from url=(${lengthOfURL})${URL}

BUG=503217
TEST=webkit_unit_tests --gtest_filter=PageSerializerTest.markOfTheWebDeclaration
TEST=webkit_unit_tests --gtest_filter=WebPageSerializerTest.fromUrlWithMinusMinu

Review URL: https://codereview.chromium.org/1371323003

Cr-Commit-Position: refs/heads/master@{#351736}
bool SerializerMarkupAccumulator::shouldIgnoreAttribute(const Attribute& attribute)
{
    PageSerializer::Delegate* delegate = m_serializer->delegate();
    if (delegate)
        return delegate->shouldIgnoreAttribute(attribute);

    return MarkupAccumulator::shouldIgnoreAttribute(attribute);
}

bool SerializerMarkupAccumulator::shouldIgnoreAttribute(const Attribute& attribute)
{
    PageSerializer::Delegate* delegate = m_serializer->delegate();
    if (delegate)
        return delegate->shouldIgnoreAttribute(attribute);

    return MarkupAccumulator::shouldIgnoreAttribute(attribute);
}
