fix(live-announcer): avoid triggering a reflow when reading directive content

[crisbeto]

Uses textContent, instead of innerText, to avoid triggering a reflow when reading off the content of the directive element.

[devversion]

LGTM. Technically a breaking change because innerText and textContent have a different value in some cases (see here)

[crisbeto]

I think the cases where the values would be different are pretty rare since you wouldn’t apply the directive on something with complex content.

[devversion]

True. That's why I wrote that it could be technically a breaking change. Would be probably still worth adding a small note for it to the changelog if we merge it.

[jelbourn]

LGTM

[ngbot]

Hi @crisbeto! This PR has merge conflicts due to recent upstream merges.
Please help to unblock it by resolving these conflicts. Thanks!

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