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
diff --git a/chrome/browser/browser_resources.grd b/chrome/browser/browser_resources.grd
index 0435551..43eaafe 100644
--- a/chrome/browser/browser_resources.grd
+++ b/chrome/browser/browser_resources.grd
@@ -24,6 +24,11 @@
         <structure name="IDR_ABOUT_WELCOME_HTML" file="resources\about_welcome_android\about_welcome_android.html" flattenhtml="true" type="chrome_html" />
         <structure name="IDR_ABOUT_WELCOME_CSS" file="resources\about_welcome_android\about_welcome_android.css" flattenhtml="true" type="chrome_html" />
       </if>
+      <if expr="pp_ifdef('enable_app_list')">
+        <structure name="IDR_APP_LIST_START_PAGE_CSS" file="resources\app_list\start_page.css" flattenhtml="true" type="chrome_html" />
+        <structure name="IDR_APP_LIST_START_PAGE_HTML" file="resources\app_list\start_page.html" flattenhtml="true" type="chrome_html" />
+        <structure name="IDR_APP_LIST_START_PAGE_JS" file="resources\app_list\start_page.js" flattenhtml="true" type="chrome_html" />
+      </if>
       <if expr="pp_ifdef('chromeos')">
         <structure name="IDR_DEMO_USER_LOGIN_HTML" file="resources\chromeos\login\demo_user_login.html" flattenhtml="true" type="chrome_html" />
         <structure name="IDR_DEMO_USER_LOGIN_JS" file="resources\chromeos\login\demo_user_login.js" flattenhtml="true" type="chrome_html" />