[ScopedTaskEnvironment] Fix UBSan issue calling a member method from...

[ScopedTaskEnvironment] Fix UBSan issue calling a member method from ScopedTaskEnvironment's initializer list

TBR=fdoray@chromium.org

Bug: 946657, 983290
Change-Id: I3e83611141f364b1d7e3a42442d896bdf6956877
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1700269Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#677094}

base/test/scoped_task_environment.h

@@ -323,7 +323,7 @@ class ScopedTaskEnvironment {
   // ScopedTaskEnvironment's constructor. Helper for the migration (while
   // TimeSource is optionally defined by MainThreadType or NowSource).
   template <class... ArgTypes>
-  constexpr TimeSource TimeSourceForTraits(ArgTypes... args) {
+  static constexpr TimeSource TimeSourceForTraits(ArgTypes... args) {
     const auto explicit_time_source =
         trait_helpers::GetOptionalEnum<TimeSource>(args...);
     const auto explicit_main_thread_type =