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The ax_tree_id retrieved from the main frame can change since the
main frame may be swapped by the webview due to navigation
requests.  This CL uses the cast web content's tab id to identify
the tab from which the current main frame can be retrieved and
(hence the ax tree id).

Bug: NONE
Test: Display assistant build with weather webview screen
Change-Id: I0e8a23a79271a853652b2dcf09dfcf4403f94b97
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2325030Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Commit-Queue: Randy Rossi <rmrossi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792788}