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Portals: Deflake browser tests

Previously, in multiple instances, we used a navigation observer to wait
for the first portal navigation with the assumption that navigation wasn't
already complete. This assumption was incorrect, the portal could navigate
before we return from the ExecJs call that initiated the navigation. This
CL creates the observer before calling ExecJs

Bug: 990717, 998107
Change-Id: I8d73b2c39e343e625fa9423291568af2c69e35cd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1873239Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: Adithya Srinivasan <adithyas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709009}