Code:  WebString WebPageSerializer::generateMarkOfTheWebDeclaration(const WebURL& url)
 {
    return String::format("\n<!-- saved from url=(%04d)%s -->\n",
                          static_cast<int>(url.spec().length()),
                          url.spec().data());
 }

Commit Message: 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}
CWE ID: CWE-20
 WebString WebPageSerializer::generateMarkOfTheWebDeclaration(const WebURL& url)
 {
    StringBuilder builder;
    builder.append("\n<!-- ");
    builder.append(PageSerializer::markOfTheWebDeclaration(url));
    builder.append(" -->\n");
    return builder.toString();
 }
