Have ElementTraversal::firstWithin() take a reference

Have ElementTraversal::firstWithin() take a reference

Have ElementTraversal::firstWithin() take a reference instead of a pointer
as its implementation expects the pointer to be non-null. This makes the
code look safer.

R=adamk

[adamk, 2013-11-12 23:50:00]

lgtm

https://codereview.chromium.org/65303008/diff/1/Source/core/dom/TreeScope.cpp
File Source/core/dom/TreeScope.cpp (right):

https://codereview.chromium.org/65303008/diff/1/Source/core/dom/TreeScope.cpp...
Source/core/dom/TreeScope.cpp:280: ASSERT(rootNode());
Sure would be nice if we could get rid of the bizarre case where rootNode() is
null (only happens for a single special instance of TreeScope that's used in a
few places). I guess this ASSERT at least points out that that's a reasonable
assumption.