Add comments to ElementData.h
Add comments to ElementData.h

Document what ElementData, ShareableElementData and UniqueElementData are.

BUG=334858

Review URL: https://codereview.chromium.org/141593003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@165960 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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