Automatically determine if a struct/class needs tracing.

This is an alternate automatic way to determine if a class or struct needs to be traced based on it having a method with the signature
"void trace(Visitor*)".

It is not a beauty, but it will avoid us having to specialize the NeedsTracing trait for structs being used in collections, such as CSSGradientColorStop.

R=ager@chromium.org, erik.corry@gmail.com, haraken@chromium.org, jochen@chromium.org, oilpan-reviews@chromium.org, tkent@chromium.org, vegorov@chromium.org, zerny@chromium.org
BUG=

Review URL: https://codereview.chromium.org/179943003

git-svn-id: svn://svn.chromium.org/blink/trunk@168143 bbb929c8-8fbe-4397-9dbb-9b2b20218538