app_list: Initial version of the start page.

- Add a StartPageService PKS that holds the start page contents and
  app recommendation generator;
- Implement the initial app recommendation using the 4 most recent apps;
- Expose the start page contents in AppListViewDelegate;
- Update AppsGridView could to handle start page case;
- Implement a WebUI for the start page contents;
- Put the start page feature behind a flag;
- Add a switch to override the start page url for easier mocking;

BUG=268660,268661

Review URL: https://codereview.chromium.org/25152002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226940 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226940