Add more assertion to render_text_harfbuzz.

Add more assertion to render_text_harfbuzz.

Crashes are reported, but single CHECK_NE isn't helpful because
the variable actually comes from either of two -- script_item_break
and style.

Things will become clearer by adding a CHECK for script_item_break.

BUG=403892
R=oshima@chromium.org, msw@chromium.org

Committed: https://crrev.com/13db1406f0aa142c8b2e777d080b76bc08d49896
Cr-Commit-Position: refs/heads/master@{#360473}

[Jun Mukai, 2015-11-18 22:42:00]

The CQ bit was checked by mukai@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-11-18 22:42:47]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1456183002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1456183002/1

[msw, 2015-11-18 23:23:48]

lgtm

[oshima, 2015-11-18 23:25:54]

lgtm

[commit-bot: I haz the power, 2015-11-19 00:31:38]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-11-19 00:31:39]

Dry run: This issue passed the CQ dry run.

[Jun Mukai, 2015-11-19 00:36:59]

The CQ bit was checked by mukai@chromium.org

[commit-bot: I haz the power, 2015-11-19 00:38:25]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1456183002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1456183002/1

[commit-bot: I haz the power, 2015-11-19 01:51:05]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-11-19 01:52:06]

Patchset 1 (id:??) landed as
https://crrev.com/13db1406f0aa142c8b2e777d080b76bc08d49896
Cr-Commit-Position: refs/heads/master@{#360473}