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The API was previously callback-based, instead of Promise-based.
Callback-based APIs are more tedious to use. As a concrete example,
the PDFExtensionJSTest.ParamsParser tests were incorrectly not waiting
for any of the callbacks to execute before declaring the test
successful.

In the process also
 - Changed Navigator#navigate() to return a Promise, instead of
   firing a 'navigate-for-testing' event.
 - Fixed incorrect type annotation for OpenPdfParams#viewPosition.

Bug: None
Change-Id: Ib14b6499d3c661bb62c21c1836324ceecf1494ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2618219
Commit-Queue: Daniel Hosseinian <dhoss@chromium.org>
Reviewed-by: Daniel Hosseinian <dhoss@chromium.org>
Auto-Submit: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842168}