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Reland explanation: A test I wrote tested specific size values for the semantic
tree. The chrome a11y code that creates the tree for us is non-deterministic, so
we can't check for a specific tree size. I updated the tests to only test for a
minimum threshold, which is fine in this case here.



This is a reland of fdae2298

Original change's description:
> [fuchsia][a11y] Sends full semantic tree when semantics are enabled.
>
> This change fixes the bug where chrome was sending the full semantic tree only for the first time that semantics were enabled. This happened because the semantic tree was not getting cleared on the chrome side when disabled. when semantic were disabled / enabled, the events fired on the tree included only the diffs from the previous state of the tree.
>
> Test: web_engine_browsertests
>
> Bug: fuchsia:59680, 1134147
> Change-Id: I8266d6436f9bfb9ee9e89a0f46c0916bac7a5e75
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2442646
> Commit-Queue: Lucas Radaelli <lucasradaelli@google.com>
> Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
> Reviewed-by: Sharon Yang <yangsharon@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#813771}

Bug: fuchsia:59680
Bug: 1134147
Change-Id: I606b7628c480b775c04d84894faaba73f74789c9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2450704
Commit-Queue: Lucas Radaelli <lucasradaelli@google.com>
Auto-Submit: Lucas Radaelli <lucasradaelli@google.com>
Reviewed-by: Sharon Yang <yangsharon@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813962}