Initial page load is not AMP with media tags filter.

When a media item is entered using the media_wysiwyg module and the "Convert Media tags to markup" filter is enabled, the initial page load of an AMP page is not an AMP page.

I have created a test that intentionally fails to highlight this. The patch with the test can be found below or in comment #13.

Steps to replicate:

1. Set up AMP as per AMP's module page. (Enable module, an AMP theme, enable AMP in a content type's display.
2. Enable the WYSIWYG Media module and set up a WYSIWYG (ckeditor) profile with the Media Browser button/plugin.
3. Add the "Convert Media tags to markup" filter in a text format.
4. Create a content item of the type above. Save and view the AMP version or add ?amp to the URL of the page.

Expected:
An AMP themed page.

Result:
The page uses the default theme.

This can also be replicated by clearing the cache and viewing the AMP URL. If you delete the media element then the page is AMP themed successfully on initial page load.

Hypothesis:
The media element causes theme hooks to be called before $custom_theme = &drupal_static(__FUNCTION__); is set within menu_get_custom_theme(). This means that while menu_get_custom_theme() is initially called with $initialize = TRUE, the hooks end up calling menu_get_custom_theme($initialize = FALSE) which results in the default theme being used.