Un-inline Element's constructor to avoid bloating

Un-inline Element's constructor to avoid bloating, as advised by Chromium
coding style:
http://www.chromium.org/developers/coding-style/cpp-dos-and-donts#TOC-Stop-inlining-constructors-and-destructors.

The Element class is used in inheritence and has non-POD data members. The
Element constructor is thus non-trivial.

This reduces release binary size by ~12Kb. I saw no impact on the Dromaeo
score:
http://dromaeo.com/?id=222265,222266

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/309593004

git-svn-id: svn://svn.chromium.org/blink/trunk@175213 bbb929c8-8fbe-4397-9dbb-9b2b20218538