Don't wait for the extensions cache to become ready on chromeos=1 builds.

The cache depends on an external component to become ready on ChromeOS.
This component isn't present on desktop builds, and so the cache never
becomes ready and the ExtensionUpdater never begins fetching external
extensions.

BUG=344839

Review URL: https://codereview.chromium.org/172163004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253851 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253851
MediaStreamDispatcherHost does not need all of ResourceContext.

Instead of passing ResourceContext, just pass the
ResourceContext::SaltCallback which is used by MSDH.

BUG=347302

Review URL: https://codereview.chromium.org/177313006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253852 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253852