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Now that support for the GET_ALLOWED_ORIGINS request is optional WebUSB
devices may only support the GET_URL request used for reading the
landing page from the device. This patch fixes the descriptor reading
logic so that this is supported.

(Note, the logic to send the GET_ALLOWED_ORIGINS request will be removed
entirely as part of issue 711443.)

BUG=721546

Review-Url: https://codereview.chromium.org/2876463008
Cr-Commit-Position: refs/heads/master@{#471141}