Make SimpleDefinitionFinder crawl JSDoc of record types in externs

Fixes #599 on github
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=75394242
Consider var declarations without a RHS in externs to be a "definition"
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=75406129
Remove RHS in extern definitons of nullable functions.

This preserves all the typing information, but avoids a warning in the
new type inference.
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=75419708