Merge ShadowRootRareData into ShadowRoot.

Merge ShadowRootRareData into ShadowRoot.

In Sky most elements with a ShadowRoot will have rare data since they'll
have elements inside them with more ShadowRoots which would increment
the m_childShadowRootCount field, or they'll contain a <content> element
which would increment m_descendantContentElementCount. Only leaf elements
that don't contain either will not get one, and everyone pays the price
for the rare data ptr.

This should be mostly net netural for memory usage and it makes the code
simpler. We can add it back later if we realize there's lots of leaf
widgets.

R=abarth@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/dad6d7f3e3edd4da67e255de631f4f3fc1ce8e22

[esprehn, 2015-01-22 01:58:09]

esprehn@chromium.org changed reviewers:
+ abarth@chromium.org, ojan@chromium.org

[abarth-chromium, 2015-01-22 02:03:53]

lgtm

[esprehn, 2015-01-22 19:47:38]

Committed patchset #1 (id:1) manually as
dad6d7f3e3edd4da67e255de631f4f3fc1ce8e22 (presubmit successful).