Fix a compile warning caused by an incomplete externs type definition
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=50490671
Add chrome.mediaGalleries.getMediaFileSystemMetadata to the chrome extensions externs. Defined at http://developer.chrome.com/apps/mediaGalleries.html#method-getMediaFileSystemMetadata.
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=50521597