Run the collectors on the UI thread temporarily

The perf collector issues dbus commands to run perf as root but the dbus client
is not thread safe.
Temporarily, map the collector task runner to the current thread (UI), until we
can implement a fix in the dbus client.

BUG=b:139816773

Change-Id: I02766ed512a10f1868dda7bd58205397997ef9e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1764319Reviewed-by: Gabriel Marin <gmx@chromium.org>
Reviewed-by: Mike Wittman <wittman@chromium.org>
Commit-Queue: Gabriel Marin <gmx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#689163}

chrome/browser/metrics/perf/metric_provider.cc

@@ -8,6 +8,7 @@
 #include "base/metrics/histogram_functions.h"
 #include "base/task/post_task.h"
 #include "base/task/task_traits.h"
+#include "base/threading/sequenced_task_runner_handle.h"
 #include "content/public/browser/browser_task_traits.h"
 #include "content/public/browser/browser_thread.h"
 #include "third_party/metrics_proto/sampled_profile.pb.h"
@@ -31,15 +32,12 @@ using MetricCollector = internal::MetricCollector;
 MetricProvider::MetricProvider(std::unique_ptr<MetricCollector> collector)
     : upload_uma_histogram_(std::string(kUploadCountHistogramPrefix) +
                             collector->ToolName()),
-      // Run the collector at a higher priority to enable fast triggering of
-      // profile collections. In particular, we want fast triggering when
-      // jankiness is detected, but even random based periodic collection
-      // benefits from a higher priority, to avoid biasing the collection to
-      // times when the system is not busy. The work performed on the dedicated
-      // sequence is short and infrequent. Expensive parsing operations are
-      // executed asynchronously on the thread pool.
-      collector_task_runner_(base::CreateSequencedTaskRunner(
-          {base::ThreadPool(), base::TaskPriority::USER_VISIBLE})),
+      // Run the collectors on the UI thread temporarily, because the perf
+      // collector must issues dbus commands and the dbus client is not thread
+      // safe.
+      // TODO(b/139818457): Move back to a dedicated sequence once the dbus
+      // client is thread safe.
+      collector_task_runner_(base::SequencedTaskRunnerHandle::Get()),
       metric_collector_(std::move(collector)),
       weak_factory_(this) {
   metric_collector_->set_profile_done_callback(base::BindRepeating(