[multi-instance] Call OpenURL from source web contents rather than on browser for opening links in a split view.

When the call is handled by the browser directly, it bypasses the need
to go through the web contents delegate and doesn't notify observers as
a result. This is now similar to how "Open link in new tab" should
behave.

Caught during implementation of daisy chaining the glic side panel.

Change-Id: I90b0ea3406e609f2189efabff8b95036cc377932
Bug: 448700332
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7005048
Reviewed-by: Bryant Chandler <bryantchandler@chromium.org>
Reviewed-by: Darryl James <dljames@chromium.org>
Commit-Queue: Sana Akbani <sanaakbani@google.com>
Reviewed-by: Alison Gale <agale@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1525044}