Check that frame is attached before using its BrowserInterfaceBroker.

This CL fixes a null dereference caused by calling
LocalFrame::GetBrowserInterfaceBroker on a detached frame.

Bug: 1246684
Change-Id: Id71d42b0881ae076626dbfb1316d1efaeb01b03d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3171051


Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Tove Petersson <tovep@chromium.org>
Cr-Commit-Position: refs/heads/main@{#923347}