Add generated main document URL request job support

Main document is an invisible main entrance of an applicaiton (see:
http://anssiko.github.io/runtime/app-lifecycle.html). To support pure JavaScript
scripts main document an empty HTML file will be created to contain these
scripts. The GeneratedMainDocumentJob implementation is borrowed from Chromium
GeneratedBackgroundPageJob to handle URL request when the requeted URL matches
"_generated_main_document.html" file name.
Add main document launch support

Main document is running in background, so we need create its runtime without
application window. A new windowless property is added to Runtime class, so the
caller can specify whether an application window will be created with it, the
default value is "flase". Currently we support both launch from local path and
main document, and the main document entry has a higher launch priority.
Add main document browser test