Rename namespace to "cef_internal" for base types

This fixes duplicate symbol errors when building CEF sample applications as
part of the Chromium build. These applications pick up a link-time dependency
on base.lib via the //sandbox and/or //testing/gtest targets which then
conflict with the symbols provided by the libcef_dll_wrapper target.

include/base/internal/cef_raw_scoped_refptr_mismatch_checker.h

@@ -47,7 +47,7 @@ namespace base {
 
 // This is a base internal implementation file used by task.h and callback.h.
 // Not for public consumption, so we wrap it in namespace internal.
-namespace internal {
+namespace cef_internal {
 
 template <typename T, typename = void>
 struct IsRefCountedType : std::false_type {};
@@ -68,7 +68,7 @@ struct NeedsScopedRefptrButGetsRawPtr
                 "NeedsScopedRefptrButGetsRawPtr requires non-reference type.");
 };
 
-}  // namespace internal
+}  // namespace cef_internal
 
 }  // namespace base