Don't merge adjacent composition markers.

Don't merge adjacent composition markers.

In CJK text compositions, a common pattern is for multiple adjacent
segments to be underlined, with a small separation between each and
possibly a thickness difference (example appearance at
http://www.w3.org/TR/ime-api/images/image08.png).
DocumentMarkerController's default behavior to merge overlapping or
adjacent markers converted this to a single uniform line, so disable
the merging for composition markers.

TEST=webkit_unit_tests --gtest_filter=DocumentMarkerControllerTest.CompositionMarkersNotMerged
BUG=488628
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=202423

[aelias_OOO_until_Jul13, 2015-09-17 00:25:55]

aelias@chromium.org changed reviewers:
+ yosin@chromium.org

[aelias_OOO_until_Jul13, 2015-09-17 00:27:59]

Hi yosin@, I noticed this regression introduced by my previous patch today while
playing around with CJK IME on desktop.  PTAL.

[yosin_UTC9, 2015-09-17 01:51:13]

lgtm

Thanks for fixing!

[yosin_UTC9, 2015-09-17 01:51:16]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-09-17 01:51:53]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1347313002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1347313002/1

[aelias_OOO_until_Jul13, 2015-09-17 02:04:32]

The CQ bit was unchecked by aelias@chromium.org

[aelias_OOO_until_Jul13, 2015-09-17 02:26:54]

The CQ bit was checked by aelias@chromium.org

[aelias_OOO_until_Jul13, 2015-09-17 02:26:55]

The patchset sent to the CQ was uploaded after l-g-t-m from yosin@chromium.org
Link to the patchset: https://codereview.chromium.org/1347313002/#ps20001
(title: "Remove unnecessary toElement() call in the unit test")

[commit-bot: I haz the power, 2015-09-17 02:27:03]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1347313002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1347313002/20001

[commit-bot: I haz the power, 2015-09-17 03:31:54]

Committed patchset #2 (id:20001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=202423