[Mac] Text Suggestions Touch Bar Native Behavior

Modify the text suggestions touch bar behavior so that it replicates
MacOS. Instead of generating suggestions for and replacing an entire
word, the touch bar will do it from the beginning of the word to the
cursor. The touch bar will also not replace the trailing whitespace.

Bug: 717553
Change-Id: Ied22203a6a9f321a8a17228f39471bb7ff3a847e
Reviewed-on: https://chromium-review.googlesource.com/1212269
Commit-Queue: Sarah Chan <spqchan@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590669}
3 files changed