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This CL adds the following classes to c/b/ui/views:
 - WebUIBubbleView
     A WebView subclass that supports the ShowUI() call from
     its hosting WebUI.

 - WebUIBubbleManager
     Takes a WebUI and GURL and manages the showing / closing of the
     WebUIBubbleDIalogView and caching of the hosted WebUIBubbleView.
     Example included in this CL is management of read later's
     WebUI.

 - WebUIBubbleDialogView
     Responsible for hosting the WebView in an anchored Views bubble.

Follow up will cleanup existing WebUI bubble code from
ui/views/controls/webview and switch Tab Search to use the
WebUIBubbleManager.

Bug: 1099917
Change-Id: I8a70394fe1718f31727c9a9d2478ea860890bcb7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2501075
Commit-Queue: Thomas Lukaszewicz <tluk@chromium.org>
Reviewed-by: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822754}