ime: Add ime API for changing the composition range.

Adds a new ime API for Chrome OS and Windows that allows extensions to
change the composition range (SetCompositionFromExistingText).

TBR=benwells@chromium.org

Bug: 952757
Change-Id: Ia7ecfebb0ed7bd7ba1d098a22961d727b949fc05
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1585822
Commit-Queue: Darren Shen <shend@chromium.org>
Reviewed-by: Darren Shen <shend@chromium.org>
Reviewed-by: Shu Chen <shuchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661577}
22 files changed