Simplify the way to initialize context

(Background: This is a preparation CL for making DOMWrapperWorld::current() callable from anywhere.)

Currently, when we initialize a context, we need to call the following things in a proper order.

- V8PerContextDataHolder::install()
- V8PerContextData::create()
- V8PerContextData::init()
- gin::ContextHolder()
- gin::ContextHolder::SetContext()

However, this is more complicated than necessary. This CL refactors code so that we just need to call V8PerContextData::create() when initializing a context.

Specifically, this CL does the following refactoring:

- Move gin::ContextHolder from V8WindowShell/WorkerScriptController to V8PerContextData.
- Remove V8_STORE_PRIMORDIAL macro.
- Move the contents of V8PerContextData::init() to V8PerContextData's constructor.

BUG=341032

Review URL: https://codereview.chromium.org/180773003

git-svn-id: svn://svn.chromium.org/blink/trunk@168260 bbb929c8-8fbe-4397-9dbb-9b2b20218538