Fix memory leak in GraphicsContextState.

In GraphicsContextState::copy we use placement-new to overwrite the current
object. We need to first call the object's destructor though to not leak any
memory.

BUG=383548

Review URL: https://codereview.chromium.org/335613002

git-svn-id: svn://svn.chromium.org/blink/trunk@176106 bbb929c8-8fbe-4397-9dbb-9b2b20218538