fix: Fix loading with global "define" set to null
fix: Fix loading with global "define" set to null

It seems that some websites have a global "define" object set to null.
This caused our module loader to break when Shaka Player was loaded.

By checking for "define" to have a "function" type, rather than just
being defined at all, we can avoid this problem.

Closes #2847

Change-Id: I645f61687b9ec298bcc51c8d128398cdad3dd3b9

tvoli/shaka-player,shaka-project/shaka-player,tvoli/shaka-player,shaka-project/shaka-player,shaka-project/shaka-player,tvoli/shaka-player,tvoli/shaka-player,shaka-project/shaka-player