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}
    const SerializedResource* getResource(const char* url, const char* mimeType)
    {
        KURL kURL = KURL(m_baseUrl, url);
        String mime(mimeType);
        for (size_t i = 0; i < m_resources.size(); ++i) {
            const SerializedResource& resource = m_resources[i];
            if (resource.url == kURL && !resource.data->isEmpty()
                && (mime.isNull() || equalIgnoringCase(resource.mimeType, mime)))
                return &resource;
        }
        return nullptr;
    }

    const SerializedResource* getResource(const char* url, const char* mimeType)
    {
        KURL kURL = KURL(m_baseUrl, url);
        String mime(mimeType);
        for (size_t i = 0; i < m_resources.size(); ++i) {
            const SerializedResource& resource = m_resources[i];
            if (resource.url == kURL && !resource.data->isEmpty()
                && (mime.isNull() || equalIgnoringCase(resource.mimeType, mime)))
                return &resource;
        }
        return nullptr;
    }
