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Motivation:
Currently, we allow all mime-types except for pdf as Payment Handler
pages. This exposes payment handlers to the vulnerabilities of certain
less-maintained mime-types. Since "text/*", "image/*", "video/*",
javascript, xml, json could satisfy a majority of use cases, this CL
allowlist the supported mime-types for payment handlers.

Before, we disallowed the "application/pdf" mime-type for
payment-handler pages.

After, we allowlist the following mime-types for payment handler pages:
* text/*
* image/*
* video/*
* application/javascript
* application/xml
* application/json

Before, we applied the throttle to mainframes.

After, we apply the throttle to all frames.

Bug: 1165367, 1165392
Change-Id: Ie69e296dc2c287c38a10ed08c1c40527f941ee47
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2622871
Commit-Queue: Liquan (Max) Gu <maxlg@chromium.org>
Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843124}