Return reference from styleEngine() accessor.

Return reference from styleEngine() accessor.

The m_styleEngine member of Document is created in the constructor and
only cleared when the OwnPtr goes out of scope as Document is destroyed.

Changed the styleEngine() accessor to return a reference with an ASSERT
checking that it is never null.

Removed null checks in Document::~Document() and styleResolverChanged()
where the styleEngine cannot be null.

Removed the clearStyleResolver method from Document since it was only
used inside the Document implementation.

R=morrita@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=191302

[dglazkov, 2015-03-04 18:23:01]

dglazkov@chromium.org changed reviewers:
+ dglazkov@chromium.org

[dglazkov, 2015-03-04 18:23:02]

lgtm

[rune, 2015-03-04 18:31:23]

The CQ bit was checked by rune@opera.com

[commit-bot: I haz the power, 2015-03-04 18:33:01]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/975933002/1

[commit-bot: I haz the power, 2015-03-04 19:57:56]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=191302