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This CL moves the extension-view-manager to a shared location to be used
by other WebUIs. The immediate use case is for a new welcome page; see
linked bug for details.

Bug: 874146
Change-Id: Ieb168e61cf6ae02e04d624c92a131e9ec38fcdae
Reviewed-on: https://chromium-review.googlesource.com/1157241Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Commit-Queue: Scott Chen <scottchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590091}