Add v8::Isolate parameter to V8ValueConverter::Strategy funcs.

It'll be needed for calling the new v8::String::New* functions (the new,
non-deprecated versions).

BUG=324225
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/94623002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237761 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237761