Use a pointer type for values in DocumentOrderedMap

Use a pointer type for values in DocumentOrderedMap (OwnPtr<MapEntry>) instead
of a struct type (MapEntry). This should hopefully fix the performance
regression on Bindings/get-element-by-id micro benchmark caused by r164029 on
some platforms.

While this CL seems to have no performance impact on Linux Desktop, I see a 14%
improvement on Bindings/get-element-by-id micro benchmark on Android (tested on
Galaxy Note 3 device). This may be due to HashTable optimizations when the
empty value for ValueType is zero, or to better caching.

R=esprehn, abarth
BUG=330778

Review URL: https://codereview.chromium.org/123333003

git-svn-id: svn://svn.chromium.org/blink/trunk@164440 bbb929c8-8fbe-4397-9dbb-9b2b20218538