gin::IsolateHolder should be owned by V8PerIsolateData

Currently gin::IsolateHolder of the main thread is owned by V8PerIsolateData, and gin::IsolateHolder of a worker thread is owned by WorkerScriptController. This is inconsistent. gin::IsolateHolder of worker threads should be owned by V8PerIsolateData as well.

(c.f., gin::ContextHolder is owned by V8PerContextData.)

BUG=341032
NOTRY=true

Review URL: https://codereview.chromium.org/185653002

git-svn-id: svn://svn.chromium.org/blink/trunk@168338 bbb929c8-8fbe-4397-9dbb-9b2b20218538