DO NOT MERGE

Add a hasExtension method to ExtendableMessageNano.

It allows checking for the presence of an extension without having to
deserialize the field.

This a cherry-pick of 0f67b212cd169ccf83221273ed88504d7e3f8165 from master.

Change-Id: Id542d20274b2435b7bc2b322740e9984cb8639a1
2 files changed