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This change adds a check in the WebContentsDelevate is available before trying
to show a prompt. Currently it crashes when trying to use it via the Payment
Handler.

Change-Id: Icfd16c2c408212b3698b6cff97a9c2b3f81f820e
Bug: 1051930
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2081351
Commit-Queue: Ayu Ishii <ayui@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Reviewed-by: Liquan (Max) Gu <maxlg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746489}