base: Mark RefCountedDeleteOnSequence constructor as explicit.

The Google C++ style guide recommends explicit constructors, with a few
exceptions where implicit conversion is desirable. Implicit conversion
between TaskRunner and RefCountedDeleteOnSequence seems surprising.

This CL does not introduce any functional changes.

Change-Id: I1ccf85fa5a673e7a91dd6090676788d682890412
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2634866
Auto-Submit: Victor Costan <pwnall@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844781}

base/memory/ref_counted_delete_on_sequence.h

@@ -38,7 +38,7 @@ class RefCountedDeleteOnSequence : public subtle::RefCountedThreadSafeBase {
 
   // A SequencedTaskRunner for the current sequence can be acquired by calling
   // SequencedTaskRunnerHandle::Get().
-  RefCountedDeleteOnSequence(
+  explicit RefCountedDeleteOnSequence(
       scoped_refptr<SequencedTaskRunner> owning_task_runner)
       : subtle::RefCountedThreadSafeBase(T::kRefCountPreference),
         owning_task_runner_(std::move(owning_task_runner)) {