[TaskScheduler]: Use ScopedBlockingCall to mark blocking tasks.

This CL uses ScopedBlockingCall to mark blocking calls in /chromecast/browser/metrics.

This CL was created by replacing calls to AssertBlockingAllowed()
with instantiations of ScopedBlockingCall(MAY_BLOCK).
I kindly ask the reviewer to make sure of the following:
  - ScopedBlockingCall is instantiated in a scope with minimal CPU usage.
    If this is not the case, ScopedBlockingCall should be instantiated
    closer to the blocking call. See scoped_blocking_call.h for more
    info. Please let me know when/where the blocking call happens if this needs
    to be changed.
  - Parameter |blocking_type| matches expectation (MAY_BLOCK/WILL_BLOCK). See
    BlockingType for more info. While I assumed MAY_BLOCK by default, that might
    not be the best fit if we know that this callsite is guaranteed to block.
  - The ScopedBlockingCall's scope covers the entirety of the blocking operation
    previously asserted against by the AssertBlockingAllowed().

This CL was uploaded by git cl split.

R=bcwhite@chromium.org

Bug: 874080
Change-Id: If065cdbaa716f646caa8c4258053a2f4a089bc21
Reviewed-on: https://chromium-review.googlesource.com/1191198
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595540}

chromecast/browser/metrics/external_metrics.cc

@@ -19,7 +19,7 @@
 #include "base/sequenced_task_runner.h"
 #include "base/task/post_task.h"
 #include "base/task/task_traits.h"
-#include "base/threading/thread_restrictions.h"
+#include "base/threading/scoped_blocking_call.h"
 #include "chromecast/base/metrics/cast_histograms.h"
 #include "chromecast/base/metrics/cast_metrics_helper.h"
 #include "chromecast/browser/metrics/cast_stability_metrics_provider.h"
@@ -119,7 +119,7 @@ void ExternalMetrics::RecordSparseHistogram(
 
 int ExternalMetrics::CollectEvents() {
   DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
-  base::AssertBlockingAllowed();
+  base::ScopedBlockingCall scoped_blocking_call(base::BlockingType::WILL_BLOCK);
 
   std::vector<std::unique_ptr<::metrics::MetricSample>> samples;
   ::metrics::SerializationUtils::ReadAndTruncateMetricsFromFile(