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This CL adds TileItemViews to the experimental app list which currently
show the first 5 apps in the app list. This CL also changes the width
of the placeholder bar on the start page to align with the tiles.

BUG=349727

Review URL: https://codereview.chromium.org/297643002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@273134 0039d316-1c4b-4281-b951-d872f2087c98