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Gets and stores the current TextInputClient and selection range when the emoji dialog is opened. Inserting emoji will insert into that TextInputClient.
Because the emoji picker window closes when it loses focus, this *should* always be correct.

One edge case is if the TextInputClient disappears after the emoji picker is opened (one example of this is the launcher's search box).

Change-Id: I1fedd494a36603b252fcff3a5e9b0d8df63a004c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2626482Reviewed-by: John Palmer <jopalmer@chromium.org>
Reviewed-by: Keith Lee <keithlee@chromium.org>
Commit-Queue: Kenton Lam <kentonlam@google.com>
Cr-Commit-Position: refs/heads/master@{#845980}