Fixing input type for ImageLoader.addImage.
Fixing input type for ImageLoader.addImage.

RELNOTES: Re-declared input type of ImageLoader.addImage from "string or Image or null" to "string or HTMLImageElement", which makes more sense.

Forbidding null caused one target to fail build, fixed in CL/113514842.

I can still be easily convinced that this is a wrong direction...
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=113534663

lucidsoftware/closure-library,platinumpixs/google-closure-library,teppeis/closure-library,Medium/closure-library,codyebberson/closure-library,ChadKillingsworth/closure-library,Medium/closure-library,ChadKillingsworth/closure-library,Dominator008/closure-library,ikabirov/closure-library,Dominator008/closure-library,ikabirov/closure-library,platinumpixs/google-closure-library,ChadKillingsworth/closure-library,federkasten/closure-library,Medium/closure-library,teppeis/closure-library,google/closure-library,google/closure-library,redforks/closure-library,federkasten/closure-library,google/closure-library,google/closure-library,teppeis/closure-library,platinumpixs/google-closure-library,federkasten/closure-library,Medium/closure-library,joeltine/closure-library,lucidsoftware/closure-library,ralic/closure-library,ralic/closure-library,lucidsoftware/closure-library,redforks/closure-library,lucidsoftware/closure-library,codyebberson/closure-library,joeltine/closure-library,ikabirov/closure-library,ikabirov/closure-library,Dominator008/closure-library,ralic/closure-library,codyebberson/closure-library,joeltine/closure-library,joeltine/closure-library,Dominator008/closure-library,teppeis/closure-library,codyebberson/closure-library,redforks/closure-library,redforks/closure-library,platinumpixs/google-closure-library,google/closure-library,ChadKillingsworth/closure-library,ralic/closure-library,federkasten/closure-library