Remove incorrect deriving of CppVariant from WebNonCopyable.

CppVariant is supposed to be a copyable class and we override the default copy ctor and assignment operator for the same. It is wrongly derived from WebNonCopyable. Remove the incorrect inheritance.

R=jochen

BUG=None

TEST=None; no behavior change;

Review URL: https://codereview.chromium.org/32943007

git-svn-id: svn://svn.chromium.org/blink/trunk@160319 bbb929c8-8fbe-4397-9dbb-9b2b20218538