Export the terms 'manifest URL' and 'start URL'.

These are generally useful concepts relevant to the manifest spec but not internal details. I am exporting them because they need to be linked from the manifest-incubations spec.

index.html

@@ -161,8 +161,8 @@ <h2>
  application is launched.
  </p>
  <p>
- A manifest has an associated <dfn>manifest URL</dfn>, which is the
- [[URL]] from which the <a>manifest</a> was fetched.
+ A manifest has an associated <dfn data-export="">manifest URL</dfn>,
+ which is the [[URL]] from which the <a>manifest</a> was fetched.
  </p>
  <p>
  A [=manifest=] can have any of the following members at its root, all
@@ -759,10 +759,11 @@ <h3>
  <p>
  The [=manifest's=] <code><dfn data-export="" data-dfn-for=
  "manifest">start_url</dfn></code> member is a <a>string</a> that
- represents the <dfn>start URL</dfn> , which is <a>URL</a> that the
- developer would prefer the user agent load when the user launches the
- web application (e.g., when the user clicks on the icon of the web
- application from a device's application menu or homescreen).
+ represents the <dfn data-export="">start URL</dfn> , which is
+ <a>URL</a> that the developer would prefer the user agent load when
+ the user launches the web application (e.g., when the user clicks on
+ the icon of the web application from a device's application menu or
+ homescreen).
  </p>
  <p>
  The [=manifest/start_url=] member is purely advisory, and a user