Add AppListView::InitAsFramelessWindow() for fullscreen view.

This CL is for the fullscreen view mode of experimental applist
which will look similar to Athena's home card.
- Allow supporting to init as a non-bubble widget. Especially,
  in that case it won't have BubbleFrameView().
- Create InitContents() for the views initialization and separate
  it from bubble initialization.
- |overlay_view_| as a custom view class, to put the property
  initialization into its ctor.
- Layout() needs to centralize the contents if it's wider.

BUG=434113
R=mgiuca@chromium.org, flackr@chromium.org
TEST=manually

Review URL: https://codereview.chromium.org/734833002

Cr-Commit-Position: refs/heads/master@{#305574}

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