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As part of the Scroll-to-expand payment handler project, this patch
will embed the web-content of the payment handler web-app into the
payment-handler bottom-sheet.

This patch also adds the CustomTabToolbar view (without the coordinator
yet) as the payment-handler's toolbar.

Bug: 999196

Change-Id: I403de9c01c3598bb120f0fb51dc7872adda8c8a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1872167
Commit-Queue: Liquan (Max) Gu <maxlg@chromium.org>
Reviewed-by: Danyao Wang <danyao@chromium.org>
Reviewed-by: Matthew Jones <mdjones@chromium.org>
Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709082}