Remove usage of the UNIT_TEST macro.

It is breaking the roll of Dawn in Chromium because on ChromeOS the
macro is defined on the command-line, causing the compilation to fail
because of a macro redefinition.

Bug: None
TBR=enga@chromium.org
Change-Id: I13eebb9b40f2a5f7c1eed4e3572de3eda2db8bb6

Change-Id: I4797211f4d9ff122d992d78aeac83fd4f0585ff6
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/24580
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/common/StackContainer.h

@@ -172,13 +172,11 @@ class StackContainer {
         return &container_;
     }
 
-#ifdef UNIT_TEST
     // Retrieves the stack source so that that unit tests can verify that the
     // buffer is being used properly.
     const typename Allocator::Source& stack_data() const {
         return stack_data_;
     }
-#endif
 
   protected:
     typename Allocator::Source stack_data_;

src/tests/unittests/StackContainerTests.cpp

@@ -6,8 +6,6 @@
 
 #include <gtest/gtest.h>
 
-#define UNIT_TEST
-
 #include "common/RefCounted.h"
 #include "common/StackContainer.h"