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Instad of generating exported validateFooRequest(),
validateFooResponse() methods, a pointer to a validation
functions is added to the generated Stub and Proxy class
prototypes. For example, given an interface Foo, with
client interface FooClient:

  FooStub.prototype.validators = [validateFooRequest];
  FooProxy.prototype.validators = [validateFooResponse];

Note that if interface Foo has no callbacks, then
FooProxy.prototype.validators is an empty array.

The router validates incoming messages using these functions
and validateMessageHeader().

BUG=407181

Review URL: https://codereview.chromium.org/488173006

Cr-Commit-Position: refs/heads/master@{#292270}