Bug 1624266 -  Use IsSharedMemoryAllowed to decide whether should the CTOR SharedArrayBuffer be defined for Window; r=nika

We set opener policy from the channel of the new document when creating a
WindowGlobalChild. However, we need to check if it's crossOriginIsolated even
when the WindowGlobalChild hasn't been created for the new inner window in some
cases.

To avoid checking the unset value of opener policy from the BrowsingContext,
this patch moves the setting the opener policy right before creating a native
global for a new inner window. So that the value of opener policy should always
be correct when validating it (IsSharedMemoryAllowed).

Differential Revision: https://phabricator.services.mozilla.com/D71534