WebstoreStandaloneInstaller: refactored to make easier to use/extend.

      WebstoreStandaloneInstaller was essentially merging two different use cases into one class, using if-else branching around switches to guide the control flow through them.

This change factors out the common part of the flow (the actual download/install + UI skeleton as Template Method pattern) into an updated abstract WebstoreStandaloneInstaller, and the differing parts of the flow (mostly specifics of the UI) into two concrete subclasses: WebstoreStartupInstaller for installs requested from the command line (i.e. without access to the browser UI), and WebstoreInTabInstaller for installs requested by a tab (with access to the browser UI).

BUG=149039


Review URL: https://chromiumcodereview.appspot.com/12386096

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@188252 0039d316-1c4b-4281-b951-d872f2087c98