[PDF Viewer] Make parsing view params async

Change parseViewParam_() and parseNameddestViewParam_() to be async.
This is needed in crrev.com/c/4371251 when we must make an asynchronous
call to fetch information from the PDF plugin.

Also add parameters to comments.

Bug: 1414864
Change-Id: I4bf3a0efa79c05b771acd2f67a655b97c688e715
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4376058


Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Andy Phan <andyphan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1123195}