Workaround for setComposition styling clobber

Workaround for setComposition styling clobber

In order to keep the previous text formatting when continuing typing on
the word, this CL detects when the beginning of the IME commit is
the same at what was there already, and restricts the write to only
the character(s) at the end that actually changed. (This CL is based on
aelias@'s CL: https://codereview.chromium.org/2073863003)

BUG=620549
Review-Url: https://codereview.chromium.org/2372493002
Cr-Commit-Position: refs/heads/master@{#424716}
(cherry picked from commit 993074d17afee685ced99283d7ffbe809fdb6f99)

Committed: https://chromium.googlesource.com/chromium/src/+/518012b2b00f0296b957476c797528a9ea03391c

[Changwan Ryu, 2016-10-17 01:28:14]

Description was changed from

==========
Workaround for setComposition styling clobber

In order to keep the previous text formatting when continuing typing on
the word, this CL detects when the beginning of the IME commit is
the same at what was there already, and restricts the write to only
the character(s) at the end that actually changed. (This CL is based on
aelias@'s CL: https://codereview.chromium.org/2073863003)

BUG=620549

Review-Url: https://codereview.chromium.org/2372493002
Cr-Commit-Position: refs/heads/master@{#424716}
(cherry picked from commit 993074d17afee685ced99283d7ffbe809fdb6f99)
==========

to

==========
Workaround for setComposition styling clobber

In order to keep the previous text formatting when continuing typing on
the word, this CL detects when the beginning of the IME commit is
the same at what was there already, and restricts the write to only
the character(s) at the end that actually changed. (This CL is based on
aelias@'s CL: https://codereview.chromium.org/2073863003)

BUG=620549

Review-Url: https://codereview.chromium.org/2372493002
Cr-Commit-Position: refs/heads/master@{#424716}
(cherry picked from commit 993074d17afee685ced99283d7ffbe809fdb6f99)

Committed:
https://chromium.googlesource.com/chromium/src/+/518012b2b00f0296b957476c7975...
==========

[Changwan Ryu, 2016-10-17 01:28:16]

Committed patchset #1 (id:1) manually as
518012b2b00f0296b957476c797528a9ea03391c.