Split the private webstore install API into two parts.

      The first part, beginInstall, must be called during a user gesture. The second
part should supply a matching id and actually starts the download and install
process.

BUG=61954
TEST=Requires webstore server side changes before it can be fully tested. For 
now, you can test that visiting the webstore and running something like the 
following command in the javascript console generates an error:

chrome.webstorePrivate.beginInstall(&quotmihcahmgecmbnbcchbopgniflfhgnkff&quot)


Review URL: http://codereview.chromium.org/4727001

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@65722 0039d316-1c4b-4281-b951-d872f2087c98