DEBUG_STRING helper macro for DisplayItem::typeAsDebugString()
DEBUG_STRING helper macro for DisplayItem::typeAsDebugString()

- Helps keep consistency of the debug strings;
- Ensures "Drawing" prefix of all drawing types.

Review URL: https://codereview.chromium.org/1117723002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@194683 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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