ime: Send surrounding text changes to IME service.

Add Mojo and protobuf messages for Chrome to send surrounding text
changes to the IME service.

Change-Id: I0ff2fdb302c884be2ff9640d7990bdfcba8d87ca
Bug: b/161490915
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2422021


Reviewed-by: Keith Lee <keithlee@chromium.org>
Reviewed-by: Alex Gough <ajgo@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820065}