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With this change, WebLayerShell now only depends on the weblayer/public
dir, and has no other dependencies on chrome code. A new APK
WebLayerSupport has all the implementation and must be installed
alongside WebLayerShell for it to work.

Change-Id: Iab3b346831c8d8737a0b3ca509455b7499b31ac9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1779428Reviewed-by: Richard Coles <torne@chromium.org>
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693775}