Add support for converting cc::Region to and from protobuf.

cc/base/DEPS

-# Things within cc/base should not depend on things in cc/ outside of cc/base.
+# Things within cc/base should not depend on things in cc/ outside of cc/base
+# except protocol buffers.
 include_rules = [
   "-cc",
   "+cc/base",
+  "+cc/proto",
 ]
 
 # Tests can use things in cc/test
 specific_include_rules = {
   ".*unittest\.cc": [
     "+cc/test",
   ]
 }