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 WebPageSerializer::serialize(WebLocalFrame* frame,
                                  WebPageSerializerClient* client,
                                  const WebVector<WebURL>& links,
                                  const WebVector<WebString>& localPaths,
                                  const WebString& localDirectoryName)
{
    WebPageSerializerImpl serializerImpl(
        frame, client, links, localPaths, localDirectoryName);
    return serializerImpl.serialize();
}

bool WebPageSerializer::serialize(WebLocalFrame* frame,
                                  WebPageSerializerClient* client,
                                  const WebVector<WebURL>& links,
                                  const WebVector<WebString>& localPaths,
                                  const WebString& localDirectoryName)
{
    WebPageSerializerImpl serializerImpl(
        frame, client, links, localPaths, localDirectoryName);
    return serializerImpl.serialize();
}
