Make sure we're on the media grid page before calling wp.media().

Merges [29725] to the 4.0 branch.

props azaozz.
fixes #29520.

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Create new $wp_roles object in install_blog() to avoid protected _init() call

Merges [30242] to the 4.0 branch.

Our call to `$wp_roles->_init()` relied on the `__call()` method in `WP_Roles` to handle the link to the protected method. This works back to PHP 5.2.9, when a bug was fixed allowing access to protected methods through this exact approach.

`install_blog()` needs a fresh `$wp_roles` object after `populate_roles()` resets everything in its path. We can create this new object from scratch, effectively doing the same thing with the call to `_init()` via the constructor.

props jeremyfelt.
fixes #29692.

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git-svn-id: http://core.svn.wordpress.org/branches/4.0@30255 1a063a9b-81f0-0310-95a4-ce76da25c4cd