bindings: Introduces ScriptWrappable::associateWithWrapper in addition to wrap.

generate_constructor_wrapper macro in bindings/templates/methods.cpp
https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit/Source/bindings/templates/methods.cpp&q=%22macro%20generate_constructor_wrapper%22%20file:methods.cpp&sq=package:chromium&l=611
would like to reuse the existing wrapper object, and in that case,
it needs to associate the impl with the existing wrapper.
(Note that wrap method creates a new wrapper.)

This CL introduces ScriptWrappable::associateWithWrapper to allow it.
This method does not need to be virtual because it must be called from
only a) wrap method, or b) V8T::constructorCallback, where we already
know the right WrapperTypeInfo.

BUG=235436

Review URL: https://codereview.chromium.org/542113003

git-svn-id: svn://svn.chromium.org/blink/trunk@181548 bbb929c8-8fbe-4397-9dbb-9b2b20218538