[Extensions] Remove ExtensionWebUI

ExtensionWebUI only did two things:
- Set the link transition type for the bookmark manager to
  PAGE_TRANSITION_AUTO_BOOKMARK.
- Instantiated the BookmarkManagerPrivateDragEventRouter.

Neither of these things should require WebUI, and having extension
WebContents have WebUI is bad.

Instead, handle the link transition in OverrideOpenUrlParams (as well
as modifying the referrer and the renderer-initiated, as we
currently do for WebUI navigations), and make the
BookmarkManagerPrivateDragEventRouter a WebContentsUserData
instantiated from extensions::TabHelper.

BUG=659798
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_site_isolation

Review-Url: https://codereview.chromium.org/2468673003
Cr-Commit-Position: refs/heads/master@{#430940}
17 files changed