Add chrome.webstore API methods to allow sites to see progress of installation

      Add two chrome.webstore API methods for progress updates:
  chrome.webstore.setInstallStageListener(function(string stage))
  - stage is either "downloading" or "installing"
  chrome.webstore.setDownloadProgressListener(function(int percent_downloaded))
Update InstallObserver to have methods for install stages.
Make WebstoreInstaller provide more frequent download updates (current consumers
like WebstoreResult imply this was intended from the start).
BUG=308634

Review URL: https://codereview.chromium.org/175263003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@259483 0039d316-1c4b-4281-b951-d872f2087c98