views: Implement SetCompositionFromExistingText.

SetCompositionFromExistingText is used by Chrome OS and Windows. We use
the existing methods on TextfieldModel and RenderText to implement this.

Bug: 952757
Change-Id: I9137d1afe70fd0bb42087c1a6842a5047a7e2f97
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1619528
Commit-Queue: Darren Shen <shend@chromium.org>
Auto-Submit: Darren Shen <shend@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661583}
4 files changed