GC plugin: remove mixin trace override restriction.

tools/clang/blink_gc_plugin/tests/class_multiple_trace_bases.txt

 In file included from class_multiple_trace_bases.cpp:5:
-./class_multiple_trace_bases.h:22:1: warning: [blink-gc] Class 'Derived' requires a trace method.
-class Derived : public Base, public Mixin {
+./class_multiple_trace_bases.h:27:1: warning: [blink-gc] Class 'Derived1' requires a trace method.
+class Derived1 : public Base, public Mixin1 {
 ^
-./class_multiple_trace_bases.h:22:17: note: [blink-gc] Untraced base class 'Base' declared here:
-class Derived : public Base, public Mixin {
-                ^
-./class_multiple_trace_bases.h:22:30: note: [blink-gc] Untraced base class 'Mixin' declared here:
-class Derived : public Base, public Mixin {
-                             ^
-./class_multiple_trace_bases.h:22:1: warning: [blink-gc] Class 'Derived' which inherits from GarbageCollectedMixin must locally declare and override trace(Visitor*)
-class Derived : public Base, public Mixin {
+./class_multiple_trace_bases.h:27:18: note: [blink-gc] Untraced base class 'Base' declared here:
+class Derived1 : public Base, public Mixin1 {
+                 ^
+./class_multiple_trace_bases.h:27:31: note: [blink-gc] Untraced base class 'Mixin1' declared here:
+class Derived1 : public Base, public Mixin1 {
+                              ^
+class_multiple_trace_bases.cpp:17:1: warning: [blink-gc] Base class 'Mixin2' of derived class 'Derived2' requires tracing.
+void Derived2::trace(Visitor* visitor) {
 ^
 2 warnings generated.