Define virtual destructors for mocks in list_container_unittest.cc.

Bug: 832726
Change-Id: I43d694483586e3e944f4494308b213ebf8a7bd3d
Reviewed-on: https://chromium-review.googlesource.com/1013141Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550823}

cc/base/list_container_unittest.cc

@@ -103,6 +103,7 @@ class SimpleDerivedElementConstructMagicNumberOne
   SimpleDerivedElementConstructMagicNumberOne() {
     set_value(kMagicNumberToUseForSimpleDerivedElementOne);
   }
+  ~SimpleDerivedElementConstructMagicNumberOne() override = default;
 };
 
 class SimpleDerivedElementConstructMagicNumberTwo
@@ -111,6 +112,7 @@ class SimpleDerivedElementConstructMagicNumberTwo
   SimpleDerivedElementConstructMagicNumberTwo() {
     set_value(kMagicNumberToUseForSimpleDerivedElementTwo);
   }
+  ~SimpleDerivedElementConstructMagicNumberTwo() override = default;
 };
 
 class SimpleDerivedElementConstructMagicNumberThree
@@ -119,6 +121,7 @@ class SimpleDerivedElementConstructMagicNumberThree
   SimpleDerivedElementConstructMagicNumberThree() {
     set_value(kMagicNumberToUseForSimpleDerivedElementThree);
   }
+  ~SimpleDerivedElementConstructMagicNumberThree() override = default;
 };
 
 class MockDerivedElement : public SimpleDerivedElementConstructMagicNumberOne {