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This CL fixes middle clicks on links inside a link capturing web app
scope from crashing. These were crashing due to the expectation that
the navigating tab was already attached to a browser window (which
is incorrect during the navigation throttle) causing a nullptr crash.

We should not be link capturing these navigations so this CL checks
for and early exits in these cases.

Bug: 1159666
Change-Id: I9dae80d84a44e8bf2fa9918ece8fee6d19af2ee2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2636009
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Maggie Cai <mxcai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844666}