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}
    String getSerializedData(const char* url, const char* mimeType = 0)
    {
        const SerializedResource* resource = getResource(url, mimeType);
        if (resource)
            return String(resource->data->data(), resource->data->size());
        return String();
    }

    String getSerializedData(const char* url, const char* mimeType = 0)
    {
        const SerializedResource* resource = getResource(url, mimeType);
        if (resource)
            return String(resource->data->data(), resource->data->size());
        return String();
    }
