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* Fix null pointer access when a shortcut is added via
  a manifest.  It was relying on the WebContents() being
  accessible immediately after stopping the observation of
  the WebContents.

* When adding a shortcut on the homescreen through web app
  banners, stop firing Intents to send the user to the
  home. Instead show a toast to indicate that a shortcut is
  being added to their homescreen.  The string was chosen by UI
  and is described on the bug.

* Hopefully fix the ShortcutHelperTests.

BUG=457424,303486
TEST=ShortcutHelperTest

Review URL: https://codereview.chromium.org/934183002

Cr-Commit-Position: refs/heads/master@{#317079}