Make ApplyBlockElementCommand not to modify uneditable element

Make ApplyBlockElementCommand not to modify uneditable element

The issue 515249 is caused by calling |lastPositionInNode()| with null pointer
which is result of |SplitTextCommand| with uneditable text node.

This patch avoid this situation by making |ApplyBlockElementCommand| not to
modify uneditable element in |rangeForParagraphSplittingTextNodesIfNeeded()|
member function where we split text node.

BUG=515249
TEST=LayoutTests/editing/execCommand/format-block-uneditable-crash.html

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=200238

[yosin_UTC9, 2015-08-10 06:11:01]

yosin@chromium.org changed reviewers:
+ hajimehoshi@chromium.org, tkent@chromium.org, yoichio@chromium.org

[yosin_UTC9, 2015-08-10 06:11:01]

PTAL

[yoichio, 2015-08-10 07:37:10]

lgtm

[yosin_UTC9, 2015-08-10 07:59:24]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-08-10 07:59:30]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1280263002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1280263002/1

[commit-bot: I haz the power, 2015-08-10 08:02:20]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=200238