Avoid storing pending requests in OpenPDFParamsParser.

OpenPDFParamsParser currently maintains an array of requests and assumes
they will get replied in the same order. By changing the return type of
getNamedDestinationCallback() to a promise, this CL enables
getViewportFromUrlParams() to handle the data returned by the plugin
directly, without storing all the requests inside OpenPDFParamsParser.

This CL also removes the usage of onNamedDestinationReceived() from
browser tests.

Bug: 55776,535978,748852
Change-Id: If6e8285a363d2efee169457856ca465cb498805a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2391102
Commit-Queue: Hui Yingst <nigi@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809449}