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Now that internal is always passing this value it's safe to remove the
default nullptr to avoid any possible future regressions for webview
window corner placement.

Bug: none
Test: manual
Change-Id: I465c888bc07eb6d339969ca0eefd8efdcc770df5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2016284Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Commit-Queue: Ryan Daum <rdaum@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734571}