* refactor: add ElectronBrowserContext::BrowserContexts()
* refactor: use ElectronBrowserContext::BrowserContexts() in ElectronBrowserMainParts::PostMainMessageLoopRun()
* refactor: use ElectronBrowserContext::BrowserContexts() in ElectronExtensionsBrowserClient::IsValidContext()
* refactor: use ElectronBrowserContext::BrowserContexts() in ElectronExtensionsBrowserClient::BroadcastEventToRenderers()
* refactor: move PartitionKey, BrowserContextMap private
* refactor: add ElectronBrowserContext::IsValidContext()
decouple ElectronExtensionsBrowserClient from the internals of ElectronBrowserContext