The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/1d58bc4000135f40585ea9aea8716a46aab05381

commit 1d58bc4000135f40585ea9aea8716a46aab05381
Author: Ryan Landay <rlanday@chromium.org>
Date: Wed Dec 06 03:31:36 2017

Make InputMethodController::SetComposition() set composition range from UndoStep

TypingCommand::InsertText(const String&, bool, EditingState*) is sort of messed
up right now because it calls AdjustSelectionAfterIncrementalInsertion(), which
calls FrameSelection::SetSelection(), which we want to hoist out of here (see
discussion at https://goo.gl/GFgLF3).

This CL removes AdjustSelectionAfterIncrementalInsertion's call to
SetSelection() and replaces it with some logic in
InputMethodController::SetComposition() to pull the ending selection out of the
most recent UndoStep. This sets us up to avoid an unnecessary selectionchange
event. Currently, SetComposition() does the following:

1. Selects the composition range.
2. Replaces the selection with the new text (requesting that the new text be
   left selected).
3. Converts the selection back into a composition range.

This CL sets us up to avoid actually selecting the range in step 2; instead, we
can make TypingCommand::InsertText() set it as the ending selection, and
InputMethodController can retrieve it from the UndoStep.

Bug: 790801
Change-Id: I9111af255f61c34f5bdaf27b94a5e8a659750860
Reviewed-on: https://chromium-review.googlesource.com/807151
Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Reviewed-by: yosin (OOO Dec 11 to Jan 8) <yosin@chromium.org>
Commit-Queue: Ryan Landay <rlanday@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521984}
[modify] https://crrev.com/1d58bc4000135f40585ea9aea8716a46aab05381/third_party/WebKit/Source/core/editing/commands/TypingCommand.cpp
[modify] https://crrev.com/1d58bc4000135f40585ea9aea8716a46aab05381/third_party/WebKit/Source/core/editing/ime/InputMethodController.cpp