Check nullable value before using it in ReplaceSelectionCommand::removeRedundantStylesAndKeepStyleSp

Check nullable value before using it in ReplaceSelectionCommand::removeRedundantStylesAndKeepStyleSpanInline

This patch changes removeRedundantStylesAndKeepStyleSpanInline() to check
nullable value |element->parentNode()| before using it. L529, if-statement
just after this change, checks |element->parentNode()| too.

BUG=369422
TEST=LayoutTests/editing/execCommand/insert-html-to-document-element-crash.html

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=175019

[yosin_UTC9, 2014-05-28 06:54:55]

Could you review this patch?
Thanks in advance.

The bug is caused by inserting PRE element before document element, but it was
failed because of document element doesn't have parent node. It is strange
that document element becomes null after 'InsertHTML', but this patch doesn't
address that. It is separate issue.

[Yuta Kitamura, 2014-05-28 08:48:52]

lgtm

[yosin_UTC9, 2014-05-29 01:33:55]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2014-05-29 01:34:14]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/yosin@chromium.org/300143012/1

[commit-bot: I haz the power, 2014-05-29 01:46:25]

Change committed as 175019