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}
void SerializerMarkupAccumulator::appendCustomAttributes(StringBuilder& result, const Element& element, Namespaces* namespaces)
{
    if (!element.isFrameOwnerElement())
        return;

    const HTMLFrameOwnerElement& frameOwner = toHTMLFrameOwnerElement(element);
    Frame* frame = frameOwner.contentFrame();
    if (!frame || !frame->isLocalFrame())
        return;

    KURL url = toLocalFrame(frame)->document()->url();
    if (url.isValid() && !url.protocolIsAbout())
        return;

    url = m_serializer->urlForBlankFrame(toLocalFrame(frame));
    appendAttribute(result, element, Attribute(frameOwnerURLAttributeName(frameOwner), AtomicString(url.string())), namespaces);
}

void SerializerMarkupAccumulator::appendCustomAttributes(StringBuilder& result, const Element& element, Namespaces* namespaces)
{
    if (!element.isFrameOwnerElement())
        return;

    const HTMLFrameOwnerElement& frameOwner = toHTMLFrameOwnerElement(element);
    Frame* frame = frameOwner.contentFrame();
    if (!frame || !frame->isLocalFrame())
        return;

    KURL url = toLocalFrame(frame)->document()->url();
    if (url.isValid() && !url.protocolIsAbout())
        return;

    url = m_serializer->urlForBlankFrame(toLocalFrame(frame));
    appendAttribute(result, element, Attribute(frameOwnerURLAttributeName(frameOwner), AtomicString(url.string())), namespaces);
}
