Rename DocumentSharedObjectPool to ElementDataCache

This name was chosen over a year ago in http://trac.webkit.org/changeset/136334
with the idea that it might hold anything. But as it still holds only
ShareableElementData, making its name (and associated methods) match
its usage makes the code clearer.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/148283015

git-svn-id: svn://svn.chromium.org/blink/trunk@166436 bbb929c8-8fbe-4397-9dbb-9b2b20218538