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This CL allows the CreditCardAccessoryController to be instantiated by
making it a WebContentsUserData. It also includes a few small updates
for consistency with https://chromium-review.googlesource.com/c/chromium/src/+/1614200

TBR=vasilii@chromium.org

Change-Id: I0363a6ec2b9d610b5f21b71f8a0f869170958d42
Bug: 902425
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1627922
Commit-Queue: Tommy Martino <tmartino@chromium.org>
Reviewed-by: Friedrich [CET] <fhorschig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663598}