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- we rasterize and activate a tree with active_tree_needs_first_draw;
- we don't draw the tree because we're not within the frame deadline [1]
- a commit happens meanwhile, which resets active_tree_is_ready_to_draw_ [2]

Now we can't draw, because we have a pending tree [3] and we can't
activate the pending tree, because active_tree_needs_first_draw [4];
So we don't produce a frame.
The next attempt to commit hangs the main thread, as it waits on the completion event [5].

1: https://source.chromium.org/chromium/chromium/src/+/master:cc/scheduler/scheduler_state_machine.cc;drc=51f3bdaec86288604ea075cf6e445e7ab3cbcc88;l=387
2: https://source.chromium.org/chromium/chromium/src/+/master:cc/scheduler/scheduler_state_machine.cc;drc=51f3bdaec86288604ea075cf6e445e7ab3cbcc88;l=877
3: https://source.chromium.org/chromium/chromium/src/+/master:cc/scheduler/scheduler_state_machine.cc;drc=51f3bdaec86288604ea075cf6e445e7ab3cbcc88;l=393
4: https://source.chromium.org/chromium/chromium/src/+/master:cc/scheduler/scheduler_state_machine.cc;drc=51f3bdaec86288604ea075cf6e445e7ab3cbcc88;l=420
5: https://source.chromium.org/chromium/chromium/src/+/master:cc/trees/proxy_main.cc;drc=cd2804d00d64031d9fd6397f6e9bc6093a2b2ef8;l=370

Bug: b/160032351
Change-Id: Ib89a513e2ab277742cea6c64ef14f945710c851a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2301202
Commit-Queue: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Eric Seckler <eseckler@chromium.org>
Reviewed-by: Sunny Sachanandani <sunnyps@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789745}