[Web Install] Separating naming of same-document and cross-document install

[dmurph]

It seems difficult and possibly buggy to combine these apis into one method call

• If the same-site can accept a manifest_id, then how will the cross-origin behavior work of defaulting the install_url as the manifest_id?
  • There could be some complex logic around "check if a manifest exists now, and if so, then use that & verify it with the manifest id. Otherwise load the manifest_id as a url in the background and try to install from there"
  • this is a tricky edge case that isn't obvious if the manifest wasn't properly added.

Instead can we separate these APIs into two calls, maybe

• navigator.installSelf - always installs this document. optional manifest_id argument.
• navigator.install - always is installing via a background web contents, loading the install url. Required manifest_id, optional install_url

(there can probably be better naming)

@reillyeon