Remove multiple inheritance from "unintesting call" mock classes.

Internal CL 156157936, which was published in commit
fe402c2779, introduced undefined behavior
by casting a base class (internal::{Naggy,Nice,Strict}Base<MockClass>,
using the curiously recurring template pattern) pointer to a derived
class ({Naggy,Nice,Strict}Mock<MockClass>), in the base class'
constructor. At that point, the object isn't guaranteed to have taken on
the shape of the derived class, and casting is undefined behavior.

The undefined behavior was caught by Chrome's CFI build bot [1], and
prevents rolling googletest past that commit / CL.

This commit simplifies the {Naggy,Nice,Strict}Mock class hierarchy in
a way that removes the undefined behavior.

[1] https://www.chromium.org/developers/testing/control-flow-integrity

googlemock/include/gmock/gmock-spec-builders.h

@@ -103,11 +103,6 @@ class ExpectationTester;
 // Base class for function mockers.
 template <typename F> class FunctionMockerBase;
 
-// Uninteresting call behavior mixins.
-template <typename M> class NiceMockBase;
-template <typename M> class NaggyMockBase;
-template <typename M> class StrictMockBase;
-
 // Protects the mock object registry (in class Mock), all function
 // mockers, and all expectations.
 //
@@ -408,13 +403,13 @@ class GTEST_API_ Mock {
   friend class internal::FunctionMockerBase;
 
   template <typename M>
-  friend class internal::NiceMockBase;
+  friend class NiceMock;
 
   template <typename M>
-  friend class internal::NaggyMockBase;
+  friend class NaggyMock;
 
   template <typename M>
-  friend class internal::StrictMockBase;
+  friend class StrictMock;
 
   // Tells Google Mock to allow uninteresting calls on the given mock
   // object.