Fix DCHECK for the existence of global field trial list

The DCHECK was introduced long ago in
https://chromiumcodereview.appspot.com/11522009 where it may have been
necessary, but in today's implementation the code does not rely on
ThreadTaskRunnerHandle.

The DCHECK is a proxy for verifying that a global field trial list
exists, which can be checked more directly via the value returned by
FieldTrialList::AddObserver().

The patch is motivated by DCHECK failures observed recently on ios
after the codepath has been triggered from the sync thread, due to
https://chromium-review.googlesource.com/c/chromium/src/+/1827368.

Bug: 1011088
Change-Id: I075c28f86962c88b9331290c1b57f2da8a3af755
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1844992
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721564}

components/variations/variations_http_header_provider.cc

@@ -16,7 +16,6 @@
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/string_split.h"
 #include "base/strings/string_util.h"
-#include "base/threading/thread_task_runner_handle.h"
 #include "components/variations/proto/client_variations.pb.h"
 
 namespace variations {
@@ -153,7 +152,9 @@ void VariationsHttpHeaderProvider::ResetForTesting() {
 VariationsHttpHeaderProvider::VariationsHttpHeaderProvider()
     : variation_ids_cache_initialized_(false) {}
 
-VariationsHttpHeaderProvider::~VariationsHttpHeaderProvider() {}
+VariationsHttpHeaderProvider::~VariationsHttpHeaderProvider() {
+  base::FieldTrialList::RemoveObserver(this);
+}
 
 void VariationsHttpHeaderProvider::OnFieldTrialGroupFinalized(
     const std::string& trial_name,
@@ -196,8 +197,8 @@ void VariationsHttpHeaderProvider::InitVariationIDsCacheIfNeeded() {
 
   // Register for additional cache updates. This is done first to avoid a race
   // that could cause registered FieldTrials to be missed.
-  DCHECK(base::ThreadTaskRunnerHandle::IsSet());
-  base::FieldTrialList::AddObserver(this);
+  bool success = base::FieldTrialList::AddObserver(this);
+  DCHECK(success);
 
   base::FieldTrial::ActiveGroups initial_groups;
   base::FieldTrialList::GetActiveFieldTrialGroups(&initial_groups);